Glossary
Isolate
In one line: Setting things up so you fight one enemy at a time, taking the other angles out of play.
What it means
Isolating means picking your fight: instead of walking into a spot where two or three enemies can shoot you, you use smokes, walls or positioning to see only one opponent at a time. A 1v1 is a coin flip; a 1v2 at the same instant is almost always lost.
It applies on attack (smoking one angle so you only face the other) and on defense (choosing a spot that only sees one part of the site). It is the key to clutches and to every clean entry.
An example
A site has two dangerous angles: one on the left, one at the back. The controller smokes the back angle: now whoever enters only has to check the left, and the fight is one on one.