Gantt chart terms: 35 definitions with examples
Start with task, duration, milestone and dependency. Then learn critical path and float when you need to understand what a delay will change. This glossary explains 35 terms with examples, in four groups: building the chart, linking tasks, tracking the work and handling bigger plans.
Use the alphabetical index to jump to a definition, or read the groups in order. The examples focus on small projects; the last group is there for when you need it.
Which Gantt chart terms does a small project actually need?
For a first chart, focus on tasks and durations, phases, milestones, finish-to-start dependencies, the today marker and the critical path. The table below points to the extra vocabulary that helps with specific problems.
| What is happening | Term to look up | Group |
|---|---|---|
| The task list is too big to estimate or keeps growing | Work breakdown structure, work package | Building |
| One task has to wait for another, and should move when it moves | Dependency, finish-to-start | Linking |
| Something has to wait a few days after a task ends, like paint drying | Lag (Ganttile has no lag setting: add a waiting task or move the start date) | Linking |
| Several things are late and you need to know which one matters | Critical path, total float | Tracking |
| Someone asks how late you are compared with what was promised | Baseline, slippage | Tracking |
| One person is booked on two full-time tasks in the same week | Over-allocation, resource leveling | Advanced |
| The deadline has been brought forward and the scope cannot shrink | Crashing, fast-tracking | Advanced |
In Ganttile, lists group tasks into phases, Auto schedule recalculates linked dates and Critical path highlights critical tasks. The definitions below note where a feature, such as a stored baseline, needs a separate record.
The glossary, grouped by when you meet each term
The terms are in the order you are likely to need them rather than alphabetical, because the early ones explain the later ones. Where a formal definition helps, it comes from the Association for Project Management's public glossary, reworded in plain language.
Terms you meet while building the chart
Task: a single piece of work with a start, an end and, ideally, one person responsible for it. Formal standards usually call it an activity. For example, "Hardware design" running for ten days is one task.
Duration: the time a task is planned to take, measured using its calendar. Five working days may span seven calendar days when a weekend intervenes. In Ganttile, Hide weekends switches duration counting to working days. A week-and-a-half beta can need only 20 hours of effort; see estimating task durations.
Effort: the hours of work a task needs, which can be far more or far less than its duration. Some tools call it work or the estimate. A contract review can take two weeks on the calendar and two hours of a lawyer's time.
Phase: a group of related tasks that belong to one stage of the project, drawn as a summary bar that runs from its first task to its last. Many tools call it a summary task. "Documentation", holding help articles, screenshots and release notes, is a phase. In Ganttile, lists play this role.
Milestone: a single point in time with no duration, marking a decision, a handover or a delivery, and often drawn as a diamond. "Contract signed" makes a stronger milestone than "Contract drafted", because signing is the moment the next phase is allowed to begin. Ganttile draws milestones as one-day coloured markers rather than diamonds. More on what makes a good milestone.
Deliverable: something the project hands over, such as a document, a feature or a finished room. The help articles are a deliverable; "Draft help articles" is the task that produces them. Milestones often mark the point a deliverable is accepted.
Timescale: the units across the top of the chart, usually days, weeks or months, and the thing to check before reading any bar. A ten-week plan fits on one screen in a week view and needs scrolling in a day view.
Work breakdown structure (WBS): a hierarchy that divides project scope into manageable pieces. A website redesign might contain content, design and build branches. The lowest-level work packages become one or more scheduled activities; they do not supply dates or dependencies. See turning a WBS into a Gantt chart.
Work package: a lowest-level element of a WBS, small enough to assign, estimate and track. "Pricing page copy" might require drafting, review and revision activities. One accountable owner can coordinate several contributors.
Terms you meet when linking tasks
Dependency: a relationship constraining the start or finish of one task relative to another. The client cannot review an unwritten draft: that is finish-to-start. Other links relate starts or finishes. Record real requirements and deliberate scheduling decisions, rather than linking rows just because they are adjacent. See task dependency types.
Predecessor: the task a dependency arrow comes from. Its start or finish constrains the successor, depending on the link type. In a finish-to-start link, "Build the prototype" precedes "Manufacture the first run"; other link types can allow overlap.
Successor: the task at the end of a dependency, the one whose start or finish waits on its predecessor. "Manufacture the first run" is the successor of "Build the prototype", and one task can have several successors.
Finish-to-start (FS): the successor cannot start until the predecessor finishes. It is the default and covers the large majority of links in an ordinary plan. Invitations go out once the venue is booked.
Start-to-start (SS): the successor cannot start until the predecessor starts. Proofreading a long report can start once the writing has started, working a chapter or two behind.
Finish-to-finish (FF): the successor cannot finish until the predecessor finishes. A translation cannot be completed until the text it is translating is final.
Start-to-finish (SF): the successor cannot finish until the predecessor starts. It is rare and mostly turns up in switchovers: the old phone system cannot be switched off until the new one has gone live.
Lag: an offset between the events connected by a dependency. A one-day finish-to-start lag could allow paint to dry before the next coat. Lag does not always mean both tasks are idle. Ganttile has no lag setting: show the wait as a short waiting task or by moving the successor's start date. A waiting task is the better choice when the interval needs an owner or its own estimate.
Lead: a negative lag that permits overlap. With a two-day lead on a finish-to-start link, review can start two days before drafting finishes. That can hide assumptions and create rework, so consider splitting the draft into reviewable sections instead.
Constraint: a date or limit the schedule has to respect regardless of the links, such as "must finish by" or "cannot start before". A venue that is only available from the 3rd is a constraint on the event setup. A pinned date stays put when the work before it slips, which is the point of it and also the risk.
Auto scheduling: recalculating dates from durations, dependencies, calendars and applicable constraints. A three-day delay may move successors, use up float or expose a constraint conflict. In Ganttile, enable Auto schedule for linked dates to recalculate.
Terms you meet once the project is running
Today line: a marker for the current date, usually a vertical line down the chart. Work sitting to the left of it that is still open needs an explanation, and on a chart sent to you that is the first place to look; reading a chart top to bottom uses it that way.
Progress: the amount of work reported complete. A percentage may come from accepted outputs or a judgment call, so check the measurement. Ganttile uses done or not done instead of percentages.
Baseline: the original schedule, frozen at the point the plan was agreed, so that later versions can be measured against it. If the baseline says the launch was on the 12th and the chart now says the 19th, the launch has slipped a week. Ganttile does not store a baseline, so keep a copy of the original plan by hand: export a PDF at kickoff and note the end dates of the main phases. Comparing plan against actual shows how that copy gets used.
Slippage: movement later than an agreed reference date. A Friday finish moved to the following Wednesday has slipped three working days, assuming no holidays. In earned value management, the formal term "schedule variance" has a different calculation; do not use the terms interchangeably without context.
Critical path: the sequence controlling the forecast finish. In a simple unconstrained schedule, critical tasks have zero total float. Constraints can produce positive or negative float, and several paths can be critical at once. See finding the critical path.
Total float: the amount of delay a task can absorb before the project's finish date moves, also called slack; the two words mean the same thing. In Ganttile's critical path example, counted in calendar days, the software track can slip 16 days and the content track 17 before the ship date moves. Tasks on the same branch draw on the same float, so a delay to the first leaves less room for the ones after it.
Free float: the delay a task can take before its own successor has to move, which can be much less than its total float. In the same example, "Write the software spec" has 16 days of total float but no free float, because "Build the software" starts the day after it ends. "Test the software", the last task on that branch, has all 16 days as free float.
Terms for bigger plans and tighter deadlines
Resource: anything the work needs that is in limited supply, most often a person, but also a machine, a room or a budget. Sam is a resource; so is the one test rig two teams want in the same week. Ganttile has no resource view, and its templates name the owner at the front of each task instead, as in Support: team training.
Over-allocation: booking more work onto a person or resource than fits their available time. Sam has a three-day survey and three-day report in a four-day week. Check effort and availability together; overlapping bars alone do not prove overload.
Resource leveling: resolving over-allocation by moving tasks until no one is booked beyond their limit, accepting that the end date may move as a result. Moving Sam's report draft to the following week is leveling. Fixing a double-booked week goes through the order to try things in.
Resource smoothing: the same idea restricted to moving tasks only within the float they already have, so the end date holds. Sliding the report draft three days later, inside the float that branch already has, is smoothing. The price is float: every day of it spent now is a day you no longer have when the next problem turns up.
Schedule compression: shortening the schedule without cutting scope. Crashing and fast-tracking are the two standard ways to do it, and both are aimed at tasks on the critical path, since shortening anything else does not move the end date.
Crashing: shortening critical path tasks by adding resources, such as extra people, overtime or paying for faster delivery. Paying a supplier to deliver parts a week early is crashing. The University of Waterloo's project management office notes in its guide to schedule compression that the first gains are usually cheap and the costs rise steeply after that, and that not every task gets shorter when more people work on it.
Fast-tracking: overlapping work previously planned in sequence. Starting the build before all designs are approved may shorten the schedule without adding people, but it can create rework and extra cost.
Critical chain: a scheduling approach that accounts for both task dependencies and resource limits. It manages uncertainty using a project buffer and feeding buffers where other chains join the critical chain. It is more than another name for the critical path.
Which terms are easy to mix up?
A handful of pairs get used as if they meant the same thing, and the difference usually matters at exactly the moment someone is deciding something.
- Duration and effort. A two-week task can be twenty hours of work or eighty. Plans that only record duration cannot show who is overloaded, and plans that only record effort cannot show when anything happens.
- Float and buffer. Float falls out of the logic: it is whatever room the shorter branches happen to have. A buffer is time someone added on purpose, usually just before a milestone. One shared buffer before a key date is often easier to monitor than a little padding hidden in every task.
- Milestone and deadline. A milestone marks an event; its forecast date may move with the work. A deadline is a target or required date. Keep the forecast and the target distinct when they differ. A launch day can be both, and it helps to know which one people mean.
- Dependency and sequence. Tasks often happen in an order out of habit. A dependency records a requirement or scheduling decision about related tasks. Check why the link exists before removing it; some reflect access, safety or shared resources rather than the work itself.
- Leveling and smoothing. Leveling is allowed to move the end date and smoothing is not. If smoothing cannot resolve a clash, revisit capacity, scope or the required date.
- Gantt chart and timeline. A Gantt chart is a type of timeline with task rows and bars showing their scheduled duration. Dependency arrows are useful but optional. A high-level timeline may show only phases and key events.
Questions about the vocabulary
- Why do some glossaries say activity instead of task?
- Formal project management standards use activity for a piece of scheduled work, and most Gantt chart software says task. On a chart they mean the same thing; the difference is the vocabulary of the source, not the idea.
- Is a PERT chart a kind of Gantt chart?
- No. A PERT chart is a network of boxes and arrows, usually drawn without a timescale, while a Gantt chart puts the same tasks on a calendar. They share the dependency logic, and PERT and Gantt side by side covers when each one is the better view.
- Where can I find the formal definitions?
- The Association for Project Management publishes a free online glossary covering many of the terms here. It uses the formal vocabulary, so expect words like activity, precedence network and float rather than task, arrow and room to slip.
See the terms on a chart
A definition sticks better when you can point to its example. Start with the bars and links, then compare the critical path with the shorter branches.
Breeze
Timen
