The term 'song holder' can refer to two different devices: a CASSETTE and an iPod. A CASSETTE is an 8-letter device that was popular in the 1980s and 1990s for playing music. It consists of a plastic shell containing magnetic tape, which is wound between two spools. An iPod is a 4-letter device that was introduced in 2001 and is still popular today. It is a digital media player that stores music and other audio files on a hard drive. Both devices are used to hold and play songs.