Crossover is a term applied to musical works and performers that achieve popularity with mainstream audiences beyond the usual listenership of their particular genre. In other words, a crossover song is one that breaks into the mainstream audience, despite a (usually) previous lack of exposure in it.

In some contexts the Term "crossover" can have negative connotations, implying the watering-down of a music's distinctive qualities to accommodate to mass tastes.For example, in the early years of rock and roll, many songs originally recorded by African-American musicians were re-recorded by white artists (such as Pat Boone) in a more toned-down style (often with changed lyrics) that lacked the hard edge of the original versions. These covers were popular with a much broader audience.

In practice crossover frequently results from the appearance of the music in question in a film soundtrack. For instance, Sacred Harp music experienced a spurt of crossover popularity as a result of its appearance in the 2003 film Cold Mountain, and bluegrass music experienced a revival due to the reception of 2000's O Brother, Where Art Thou?. Even atonal music, which tends to be less popular among classical enthusiasts, has a kind of crossover niche, since (as Charles Rosen has noted) it is widely used in film and television scores "to depict an approaching menace."

The largest figure to date for a crossover hit in the US has come from Grammy Award-winning country singer LeAnn Rimes, whose song "How Do I Live" sold over 3 million copies and spent a world record breaking 69 weeks on the Hot 100 chart, more than any other song in history, despite peaking only at number 2. It was also a massive hit in Europe.