Glossary
Crossfire
In one line: Two players watching the same spot from different angles: whoever walks in cannot face both.
What it means
A crossfire is a simple trap: two defenders (or two attackers after the plant) aim at the same passage from two spots far apart. Whoever walks in can only turn towards one of them; while they fight the first, the second kills them.
It works if the two angles are far enough apart that they cannot be checked together, and if the two players actually support each other: when one shoots, the other must be ready for the trade. Against a crossfire, attackers use smokes or flashes to isolate one angle at a time.
An example
On defense, you watch the site entrance from the back while your teammate watches it from the side near the exit. The first attacker comes in and turns towards you: your teammate takes them from the side. The second attacker has the same problem.