Focus Confirmation
An indicator, usually a dot or the illumination of an AF point, showing that the camera considers focus achieved.
In depth
Confirmation works even with fully manual lenses on many bodies, because the phase-detection system can measure focus error regardless of whether it can drive the lens. Its tolerance is set by the camera's assumed circle of confusion, so it confirms rather sooner than a critical viewer would, particularly at very wide apertures.
Examples in practice
An adapted vintage 50 mm lights the focus dot when turned to the right distance, giving a check on manual technique.