Aperture Priority
A semi-automatic mode, marked A or Av, in which the photographer sets aperture and ISO while the camera chooses shutter speed.
In depth
It is the mode of choice whenever depth of field is the governing decision and light is changing — portraits, landscapes, street work. The risk is the shutter speed drifting below what can be handheld, which is why setting a minimum shutter speed under Auto ISO effectively turns aperture priority into a safer hybrid.
Examples in practice
Walking a market at f/2.8 in aperture priority, the shutter speed rises and falls between stalls while the background blur stays constant.