A heartbeat is the rhythmic pulse created when your heart squeezes and relaxes to pump blood through your body. You can feel your own heartbeat by pressing two fingers gently against your wrist or the side of your neck. That steady thump-thump-thump you feel is your heart doing its essential work, delivering oxygen and nutrients to every cell in your body.

Your heartbeat speeds up when you run or feel excited, and slows down when you rest or sleep. A normal resting heartbeat for a kid is usually between 70 and 100 beats per minute, though athletes often have slower heartbeats because their hearts have grown stronger and more efficient.

The word also describes something happening incredibly fast. If someone says they'll be back in a heartbeat, they mean they'll return almost instantly. When a firefighter rushes into a burning building, we might say she did it in a heartbeat because she didn't hesitate for even a second. The phrase captures how a heartbeat is one of the fastest, most constant rhythms we experience.

People also use heartbeat to mean the central, vital force of something: “Main Street is the heartbeat of our town” means it's where the energy and life of the community are concentrated.