Define plastic hinge

A plastic hinge is a section of a flexural member where the bending moment has reached the plastic moment $M_p$, so the whole depth has yielded and the section can rotate under a constant moment $M_p$, like a hinge that carries a moment. The yielding actually spreads over a finite length of the member, called the plastic hinge length.

Plastic hinges form at points of maximum moment: fixed supports, joints, and under concentrated loads or at the section of zero shear under a UDL. When enough hinges form to turn the structure into a mechanism, it collapses.