Glossary
Retake
In one line: On defense, going back into a site the attackers took, together, to defuse the spike.
What it means
A retake is when the defense has lost a site (often with the spike already down) and comes back to win it. It is hard: the attackers are inside, in spots of their choosing, and the clock is running.
A successful retake has three ingredients: arriving together (not one at a time), using utility to force attackers off their positions, and watching the clock to know when you must defuse even without killing everyone. A 2v3 retake with no utility is sometimes better not attempted, saving your guns instead.
An example
The spike is down on A and three of you are left. You group up close by, wait for the controller's smoke and the initiator's flash, then go in together from two sides. The attackers do not know who to look at.