Reflected-light Metering
Measuring the light bouncing off a subject toward the camera, which is what every built-in camera meter does.
Measured in
EV
In depth
Because it measures reflected light, the reading depends on the subject's tone as much as the illumination. The meter has no way to know whether a dark reading means a black cat or a dim room, so it assumes everything averages to 18 per cent reflectance. Understanding that single assumption explains almost every metering error a photographer will make.
Examples in practice
A black cat and a white cat in identical light produce meter readings three stops apart, and both will be rendered grey without compensation.