WordNet sense 1 (a group of men who escort and protect some important person):

bodyguard

WordNet sense 2 (a device designed to prevent injury):

safety, safety device

WordNet sense 3 (protect against a challenge or attack):

defend, hold

WordNet sense 4 (watch over or shield from danger or harm; protect):

ward

WordNet sense 5 (a precautionary measure warding off impending danger or damage or injury etc.):

precaution, safeguard

WordNet sense 6 (the duty of serving as a sentry):

guard duty, sentry duty, sentry go