Lock-on is a feature of many radar systems that allows it to automatically follow a selected target. Lock-on was first designed for the AI Mk. IX radar in the UK, but first used extensively by US ground-based SCR-584 radar. The SCR-584 demonstrated the ability to easily track almost any airborne target, from aircraft to artillery shells.
In the post-WWII era, the term became more widely used in connection to missile guidance concepts. Many modern anti-aircraft missiles use some form of semi-active radar homing, where the missile seeker listens for reflections of the launch platform's main radar. To provide a continual signal, the radar is locked-onto the target, following it throughout the missile's flight.
In older radar systems, through the 1980s, lock-on was normally assisted by a change in the radar signal characteristics, often by increasing the pulse repetition frequency. This led to the introduction of radar warning receivers that would notice this change and provide an audible warning to the operator, typically the pilot. Missile lock-on is typically indicated to the pilot or missile operator via an audible tone, a head-up display or a helmet-mounted display.