Fitness means being in good physical condition, with a healthy, strong body that works well. When someone has good fitness, their heart, lungs, and muscles are strong enough to handle physical activities without getting exhausted too quickly. A person with good fitness might run a mile without gasping for breath, climb several flights of stairs easily, or play an active sport without needing constant breaks.

Building fitness requires regular exercise and movement. Swimming, running, playing soccer, dancing, doing push-ups, or even walking briskly all improve fitness. The more you challenge your body with physical activity, the stronger and more efficient it becomes. Someone training for a race works on their fitness by running regularly. A gymnast builds fitness through hours of practice that strengthen their muscles and improve their endurance.

Fitness includes several components: strength for lifting and pushing, speed for quick movements, flexibility for bending and stretching, balance for staying steady, and stamina for sustaining effort over time. A fit body recovers quickly from exertion and is less likely to get injured during physical activities.

The word can also describe how well-suited something is for a particular purpose. In biology, fitness describes how well an organism is adapted to survive and reproduce in its environment. A polar bear has high fitness for Arctic life because its thick fur and layer of fat help it survive in freezing temperatures.