Plain English Meaning
A landing the pilot did not plan to make, carried out because something has gone wrong in flight and getting the airplane on the ground promptly is the safest choice.
Full Definition
An unplanned landing, made on or off an airport, conducted in response to an in-flight situation that makes continued flight inadvisable or impossible. Includes forced landings (engine failure or other condition requiring immediate landing), precautionary landings (landing while the airplane is still under power but conditions make continued flight unwise), and ditching (a forced or precautionary landing on water).