Physical security protects physical access to systems and facilities — perimeter (fences, cameras), building entry (badges, guards, mantraps), and device security (cable locks, locked server rooms).
Physical security is the foundation — all technical controls fail if an attacker has physical access (cold boot, evil maid, hardware keylogger). Tailgating/piggybacking defeats badge controls. Mantraps prevent tailgating. Data center physical security: cage, locked cabinet, biometric access.