MacOS X
1. A hybrid of the original Macintosh operating system and NextStep, based on Mach and FreeBSD and designed as the native system for recent-model Mac hardware.
2. Bad-ass rock-stable OS for people who actually want to get work done and don't want to screw around with drivers for 8000 different chipsets and security holes the size of the Ted Williams Tunnel.
3. Something PC zealots completely deny the existence of.