A mainstay is something or someone that provides essential support or stability. If your school's drama program depends heavily on one dedicated teacher who directs every play, handles costumes, and trains new actors, that teacher is a mainstay of the program. Without her, everything might fall apart.

A family's mainstay might be a grandparent who keeps everyone connected, or the steady income from a parent's reliable job. In your diet, rice, bread, or pasta might be mainstays: foods you eat regularly that form the foundation of many meals. A basketball team's mainstay could be their most consistent player, someone the coach can always count on.

Mainstays are fundamental, the difference between something working well and barely working at all. They're central to whatever they support. When you hear that something is “a mainstay,” you know it's the foundation that keeps everything else running.