Plain English Meaning
The layers inside a composite part have come unstuck from each other. From the outside it can look fine, but inside it's no longer one solid piece.
Full Definition
The separation of bonded layers in a composite structure, where the resin or adhesive holding the layers together has failed, causing the layers to come apart internally even if the outer surface still looks intact.
Context Anchor
Seen during preflight inspection of outer wing surfaces, tail surfaces, and other aircraft parts made from bonded layers.
Derivation
From Latin 'de-' (apart, away from) and 'lamina' (a thin layer or plate). Composite parts are built from many thin layers bonded together, so 'delamination' literally means the layers coming apart.
Why Pilots Care
Weakens the structural strength of composite parts and can to failure if the damage spreads in .