Gusset

A gusset is a triangular (or occasionally another shape) brace fastened across the inside corner of a frame or joint, reinforcing it against racking force without requiring the joint itself to be beefed up. Plywood gussets glued and screwed across a frame corner are a common, low-cost way to stiffen a shop cabinet, workbench base, or outdoor structure's joints, especially where the corner joint itself (a simple butt or lap joint) wouldn't resist racking well on its own.