A device that detects smoke and typically sounds an alarm. Used in homes and buildings for fire safety.
The smoke detector went off, so we evacuated outside immediately.
Smoke detectors are installed in this building.
火災報知器 is a broader term for fire alarm systems, which may include manual pull stations and heat detectors, while 煙探知器 specifically detects smoke.
Compound of 煙 (smoke), 探知 (detection), and 器 (device). A straightforward modern technical term.