Terminal guidance deals with providing information to a weapon in the final part of its trajectory, or to an aircraft about to land.
Assuming three phases of flight, the terminal phase begins at the end of the midcourse, which, in turn, began at the end of the boost phase. A boost phase may be very short, as for an artillery, or longer, as for a ballistic missile. The midcourse may be extended for a cruise missile, or trivially short for a ballistic weapon from the end of boost to the apogee.
Terminal guidance may be generated by the weapon itself, as with an active radar seeker on an AIM-120 AMRAAM air-to-air missile. Alternatively, the guidance system may be semi-active, homing in on radiation reflected from a radar illuminator (e.g., the AN/SPG-62 for a RIM-156 Standard SM-2) or from a laser designator for a laser-guided bomb. Terminal guidance may be passive, as in radar seeking by the BaE Systems ALARM, or by thermal imaging on a GBU-53 Small Diameter Bomb.
Aircraft may be guided, on their final approach, by glideslope localizing radio signals or ground-controlled approach radar commands.