In TF2, to crater is to fall from a ledge at low health and be killed by the fall damage. No matter who you're playing and how it happened, Cratering is both embarrassing and extremely funny, so craters are often followed by laughter, no small part of it belonging to the player who just cratered.
Scout: on 3 health
Pyro: Airblasts scout off a ledge
*Pyro finished off Scout*
Scout's Teammate: "GUYS! DID YOU SEE HIM CRATER JUST NOW?"
Everybody, including Scout: *Dies of Laughter*
In the gameShattered Galaxy, crater describes territory 44 -- the territory in the exact middle of the map. It has only one POC, is easily defended, AND repairs can be made there without returning to the capital, making it an ideal forwards-base -- as well as a hotly-contested zone for battles.