Flex means to bend something, especially a joint or muscle. When you flex your arm, you tighten your bicep and bend your elbow. Gymnasts, dancers, and athletes flex their bodies in ways that most people can't, showing impressive strength and control. Your body has dozens of joints that can flex: your knees, fingers, wrists, and spine all bend through flexing.

The word also describes something that can bend without breaking. A flexible ruler made of plastic can curve around corners, while a wooden one would snap. Materials that flex are useful: a flexible garden hose bends to reach different plants, and flexible wire can be shaped for different purposes.

People also use flex to mean showing off, especially about money, possessions, or achievements. If someone posts pictures of expensive sneakers or brags about vacation destinations, others might say they're flexing. This usage comes from the idea of flexing muscles to show strength, except now it means displaying wealth or status. Some people consider this kind of flexing tacky or immature, especially when it seems designed to make others feel bad.

The noun flex can mean an act of showing off.

The noun flexibility describes the ability to bend or adapt, whether physically or in planning and thinking.