means A device that detects some physical condition — light, heat, motion, pressure — and converts it into a signal a machine can read.

from From Latin 'sentire,' to feel or perceive — the same root that gives us 'sense,' 'sentiment,' and 'sentinel.' 'Sensor' itself is a relatively modern back-formation, shortened from 'sensory' and pressed into service in the 20th century as machines learned to feel on our behalf. So the word literally means 'a thing that senses,' which is exactly what it does.