Plain English Meaning
How hard it is to bend something. A part with high bending stiffness flexes very little under load; a part with low bending stiffness flexes more easily.
Full Definition
A structural property describing how strongly a material or part resists bending when a load is applied across it. Bending stiffness depends on both the material itself and the shape and thickness of the part. In composite aircraft structures, it is a key reason thicker sandwich panels and certain layups are used: they resist flexing far more than a thin sheet of the same weight.
Context Anchor
Seen in discussions of composite aircraft structures, especially how layers, cores, and skins are arranged to make a part light but resistant to bending.
Derivation
From 'bending' (to flex or curve under load) and 'stiffness' (resistance to deformation). The combined term simply means 'resistance to being bent.'