In project management, float or slack is the amount of time that a task in a project network can be delayed without causing a delay to:[1]: 183 Total float is associated with the path.
The total float represents the schedule flexibility and can also be measured by subtracting early start dates from late start dates of path completion.
[3][2]: 502 [1]: 183 Float is core to critical path method, with the total floats of noncritical activities key to computing the critical path drag of an activity, i.e., the amount of time it is adding to the project's duration.
[4] Consider the process of replacing a broken pane of glass in the window of your home.
Delaying the acquisition of the glass is likely to delay the entire project - this activity will be on the critical path and have no float, of any sort, attached to it and hence it is a 'critical activity'.