Back-button focus
Technique
A setting that separates focusing and shutter release: a button on the camera back controls focus, the shutter button only takes the photo.
Normally the camera focuses when you half-press the shutter. With back-button focus you use a separate button (AF-ON or AEL) for focusing. Advantage: focus once and take multiple shots without refocusing. Or switch seamlessly between AF-S (one press) and AF-C (hold the button). Popular with sports and wildlife photographers.
Tools that help with this: