Refers to slack, looseness, or lack of tension, often in a physical or metaphorical sense. Common in business, engineering, or casual contexts.
ロープにスラックがあると危ない。
It's dangerous if there is slack in the rope.
There is slack in this rope.
プロジェクトにスラックを持たせておく。
We allow some slack in the project schedule.
Please take up the slack in the rope.
I built in a little slack in the schedule.
緊張 describes tautness or tension, the opposite state to the looseness that スラック describes.
スラックス means trousers/slacks (clothing), while スラック means slack or looseness; they sound almost identical but are unrelated in meaning.
Borrowed from English 'slack'.