The diaphragm is a large, dome-shaped muscle that sits below your lungs and does the main work of breathing. When you breathe in, your diaphragm contracts and flattens downward, pulling air into your lungs. When you breathe out, it relaxes and curves back up, pushing air out. You can't see or directly control this muscle the way you flex your arm, but it works constantly, even when you're asleep.

If you've ever gotten the hiccups, that's your diaphragm having involuntary spasms. When you laugh really hard and feel breathless, or when you're nervous and your breathing feels shallow, that's your diaphragm at work, too.

The word comes from Greek, meaning “partition” or “barrier,” which makes sense since the diaphragm separates your chest cavity from your abdomen. Singers, musicians who play wind instruments, and athletes often train to use their diaphragm more effectively. When a voice teacher tells you to “breathe from your diaphragm,” they mean using this powerful muscle to take deep, full breaths rather than shallow ones from just your chest.