In a cycle of project control , slack time refers to?

The amount of time that a schedule activity can be delayed

without delaying the early start of any immediately following

schedule activities.

Slack is the amount of time that a task in a project network can be delayed without causing a delay to:

subsequent tasks ("free float")

project completion date ("total float") 

Slack Time refers to the amount of time the activity can be delayed without affecting the project completion Date

It is the time a task can be delayed without affecting the project completion deadline.