glossary — plain english
Slack (float)
Also called: float, total float, free float
How long a task can slip before it delays anything else. Tasks on the critical path have none; everything else has some, and knowing how much is free information.
On the site-launch chart, the photo shoot runs alongside a three-week build and only takes one week. It could start late, run long, or both — up to two weeks of trouble — and the launch wouldn't move. Those two weeks are its slack. The formal term is float, and schedulers split it into free float (slip without disturbing the next task) and total float (slip without disturbing the end date), but the everyday question is just: "if this slips, does anyone downstream feel it?"
Slack is where a plan keeps its flexibility, and it's free information — the chart already knows it. A task with lots of slack can lend its people to the task that has none. A task with zero slack is, by definition, on the critical path, and every day it loses comes straight out of the deadline.
Don't confuse slack with a buffer: slack is the room a task happens to have because of where it sits; a buffer is room you deliberately built in. One is found money, the other is savings.
See it on a real chart
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