Automatic Servo Selection
A mode, marked AF-A, in which the camera decides whether to lock focus or track continuously based on whether the subject appears to be moving.
In depth
It is a convenience mode that removes a decision at the cost of predictability: the camera may switch to tracking because of a passing pedestrian, or stay locked because a subject's movement was toward the camera and hard to detect. Most experienced photographers choose explicitly instead, and many modern bodies have quietly dropped the mode.
Examples in practice
A camera in AF-A locks on a stationary child, then switches to tracking the instant they start to run.