Metering Mode
The pattern a camera uses to sample scene brightness when calculating an automatic exposure.
In depth
Every mode is a different answer to the question 'which part of this scene should be correctly exposed?'. Broad patterns are robust and lazy; narrow patterns are precise and unforgiving. Choosing a mode is really choosing how much of the exposure decision you are delegating to the camera.
Examples in practice
Matrix metering for a snapshot in even light; spot metering for a performer lit by a single follow spot.