Aperture-priority describes a metering method which automatically sets (or displays) a shutter speed, based on the aperture chosen by the user. The term typically refers to a camera offering autoexposure—for example, one where an A or Auto position is found on the shutter-speed control. But aperture priority could also describe a method of manual (or metered manual) exposure in which the photographer chooses an aperture for its optical qualities or for other reasons, and then adjusts the shutter speed to give the proper exposure. Both manual aperture priority and manual shutter-speed priority are of course possible with match-needle or center-needle cameras.
Most true autoexposure aperture priority on SLR's requires a method of transferring the aperture value from the lens to the camera, such as the external linkages on Minolta MC/MD lenses and early Nikon F-mount lenses, which were designed for metered-manual but adapted well to aperture priority with the advent of more advanced bodies.
However, there exists at least one stopped-down metering aperture priority camera, in which the lens aperture is closed by default and the camera chooses a shutter speed based on only the light coming through the lens: the Yashica TL Electro AX. The original aperture-priority SLR, the Pentax ES was capable of stopped-down metering for autoexposure as well, because most lenses available for the m42 mount at the time did not communicate the aperture as did the Takumar lenses designed for it. Several other cameras also offered this feature for compatibility's sake.
Aperture Priority may be be the preferred type of metering when control of depth-of-field is required. By specifying a small aperture, the photographer can ensure that objects at different distances are all in focus; whereas by specifying a wide aperture, the main subject of a photo can be isolated in sharp focus while the background is deliberately blurred. This is most useful in the context of fairly static subjects, where the exact shutter speed used is of less importance.
Another reason aperture priority was once more common than shutter-speed priority is because it is easier to engineer an SLR lens to communicate the aperture to the camera than to engineer a camera that can control the aperture with precision. Several of the early lens systems could adapt to aperture priority but not shutter-speed priority.
(Compare with Shutter priority.)