The flip side (sometimes spelled flipside) is, usually, the side of a 45 RPM record that doesn't contain the song the record company intended to be the single.
However, in some cases, the song on the flip side became more popular than the other song; in other cases, most notably Elvis Presley's "Don't Be Cruel" and "Hound Dog," both sides of the record ended up becoming hits.
This term is now essentially obsolete, a victim of both changing technology and changes in the music business. Record companies issue fewer songs as singles than they used to, and the singles they do issue to the general public are on cassette or CD and almost always have more than two songs.
Some 45s are still made for use in older jukeboxes, but they tend to be produced with one hit song on each side (and in many cases, the two songs aren't by the same artist).