Stereo refers to sound that comes from two or more sources to create a sense of space and direction. When you listen to music through stereo speakers or headphones, different sounds come from the left and right sides, just like how you hear the world around you with both ears. A guitar might play more loudly in your left ear while drums hit harder in your right, making you feel like you're sitting in the middle of the band.

Before stereo, most recorded music came from a single source (called mono, or monophonic sound), making everything sound flat and crowded together. Stereo sound, invented in the 1930s but popularized in the 1950s and 1960s, revolutionized how people experienced recorded music by spreading instruments and voices across space.

A stereo system is the equipment that plays this kind of sound: speakers, amplifiers, and audio sources working together. When someone says they're playing music on their stereo, they mean their sound system at home.

People also use stereo as shorthand for stereophonic, particularly when talking about recordings. An album might be released in stereo to distinguish it from older mono versions.

As an adjective, stereo describes sound or equipment that uses this left-right effect, as in “stereo speakers” or “a stereo recording.”