Auto Exposure Lock (AE-L)
A control that freezes the metered exposure so the frame can be recomposed without the reading changing.
In depth
AE lock separates the act of measuring from the act of composing, which is essential when the subject you want to meter is not where you want it in the frame. Most cameras also let AE lock be assigned to a back button and decoupled from the shutter release, so metering and focusing become independent decisions.
Examples in practice
Meter off a face in the centre, press AE-L, then recompose so the subject sits on a third — the exposure does not drift.