Definition: Describes a game where the game play is centered around not being detected, and generally going through the game primarily unnoticed. Games that fall into the stealth category generally involve allot of sneaking around, silent kills, and intrusion into building via means other than the front door, so to speak.
Examples:
In order to progress in the game you will need to use allot of stealth tactics, and make it through the majority of the missions unnoticed. A prime example of stealth games are Tom Clancey's Splinter Cell series of titles.