CTD means Crash To Desktop, a term widely used among gamers in the usenet, forums and mail lists.

A crash to the desktop when playing a game can be a disaster: imagine flying a 4 hour mission in a B-17 simulator just to have your game trashed and lost before having a chance to drop your bombs. The only cure to CTD is good and responsible programming, and a healthy PC (all the stuff Windows users know they have to get right: drivers, swap file, resident programs, etc).