A detector is a device that senses or discovers something that's hard to notice otherwise. Metal detectors at airports find hidden weapons by sensing metal objects. Smoke detectors alert you to fire by detecting tiny smoke particles in the air before you can smell them. A lie detector (also called a polygraph) tries to tell if someone is lying by measuring changes in their heartbeat, breathing, and sweating.

Detectors extend your senses beyond what your eyes, ears, or nose can pick up naturally. Scientists use radiation detectors to measure invisible rays that would be dangerous if unnoticed. Some detectors discover things far away: astronomers use sensitive detectors to find distant planets by measuring tiny changes in starlight.

A good detective detects clues that others miss. When you detect a pattern in math problems or detect that your friend seems upset, you're noticing something that isn't immediately obvious. A detector does this work automatically, watching for specific signals and alerting you when it finds them.