Plain English Meaning
Putting the airplane down on water on purpose because you can't make it to a runway or safe ground.
Full Definition
A controlled emergency landing of an aircraft on water, performed deliberately when continued flight is not possible and no suitable land surface is reachable.
Context Anchor
You will see this term in emergency landing and glide discussions, especially when a power loss or other problem leaves water as the only practical landing surface.
Derivation
From the British military slang use of 'ditch' (originally a trench or channel) to mean any body of water -- particularly the English Channel, which RAF pilots called 'the ditch.' To 'ditch' an aircraft came to mean putting it down in the water.