When you hum, you make a musical sound with your lips closed, letting the sound vibrate in your throat and nose. Try it right now: close your mouth and make a steady “mmmmm” sound while thinking of a tune. That's humming. People hum when they're happy, when they're trying to remember a song, or when they're working on something and don't even realize they're doing it.

Machines hum too. A refrigerator hums quietly in the kitchen. An electric fan hums as it spins. Computer servers hum in data centers. This kind of humming is a low, steady, continuous sound, like a musical note that doesn't stop. When you walk into a room full of computers or machinery, you might hear everything humming together, creating a constant background buzz.

As a noun, a hum is that low, steady sound itself. You might hear the hum of a fan, the hum of traffic, or the hum of computers.

A place can also be described as humming with activity when it's busy and full of energy. A classroom before the bell rings might be humming with conversation. A restaurant kitchen during dinner service hums with chefs cooking, timers beeping, and orders being called out. When something is really bustling and alive with action, people say it's humming.