Sim

A sim is essentially a server in Second Life (they actually pack 4 sims to an actual server box which is stupid but that's another story). It hosts a fuckton of land and runs physics and shit.

Sims are vulnerable to being crashed by any number of things, namely physics overload, too many particles, or delinking too much shit at one time. At the time of this writing, Sims are using Havok 1 as their physics engine but LL is about to migrate to v4 to try and fix the physics overload issue.