breach - Definitions from Dictionary.com
1. the act or a result of breaking; break or rupture.
2. an infraction or violation, as of a law, trust, faith, or promise.
3. a gap made in a wall, fortification, line of soldiers, etc.; rift; fissure.
Security is viewed as a protective wall (Dan Browns "Digital Fortress" and the term 'firewall' use this same image.) and a security breach is a hole or break in that wall.