Stirrup
A closed or hooked loop of reinforcement wrapped around the main bars in a beam or column to resist shear and hold the longitudinal steel in place.
What does stirrup mean?
Stirrups and column ties resist the diagonal tension that causes shear failure, which is sudden and brittle compared with bending failure. Their spacing tightens near supports where shear is greatest and near beam-column joints in seismic design. Hook geometry and hook length are specified precisely because a stirrup that opens under load does nothing.
In practice
The rectangular loops spaced along a grade beam or a column cage are stirrups, and their spacing is called out on the structural drawings.