Falsetto, often translated as a "false voice," is a vocal technique that allows male singers to perform notes ordinarily out of their natural range. Essentially, it pulls the male singer's voice out of the chest and into the head, which is traditionally what helps female sopranos hit their highest notes. Some male singers only use falsetto to reach a few high notes before returning to their natural chest and throat voices, but a few can actually sing entire songs using this controlled technique.
The use of falsetto has been traced back to at least the Middle Ages, although early music theorists used the term almost interchangeably with "head voice." Both men and women working in the field of opera were trained to use falsetto, although it was more common to hear trained male countertenors use it whenever female sopranos were either not available or else not permitted to perform. Male bass singers also used the technique sparingly when asked to perform notes in the high tenor range.
In modern music, the use of falsetto became very prominent during the 1950s, as a form of a capella music called "doo wop" became popular among the younger generation. Doo wop groups were almost entirely composed of a bass, baritone, lead tenor and first tenor, much like Southern gospel quartets of the time. The first tenor of a typical doo wop group often learned how to sing entirely in falsetto, which served as a melodic counterpoint to the lead tenor's straightforward delivery. While the first tenor would sing extremely high notes, the bass would counter with deep runs of his own.
A song by the Tokens, "The Lion Sleeps Tonight," featured a straight falsetto performance from beginning to end. Singer Frankie Valli spent most of his singing career using an unusually powerful falsetto, as witnessed in the song "Walk Like a Man." Other singers such as Roy Orbison would use this technique in combination with an impressive natural chest voice. Generating power and maintaining tone in the head voice is notoriously difficult, but trained rock vocalists often learn how to switch into it just before hitting the highest notes of their songs.
It is important to note that head voice and falsetto, although often used interchangeably, are two different methods of vocal production that involve totally different laryngeal articulations. Falsetto resembles chest voice articulation, using the entire length of the vocal fold (minus the glottis), except in falsetto the vocal folds do not fully come together when producing sound, allowing a greater amount of airflow, which gives the voice a breathy quality. Head voice involves "zipping up" the vocal folds part of the length to give a sort of shorter, tighter arrangement. This looks (from view of a laryngoscope) completely different from falsetto, and although some operatic schools interchange head voice and falsetto, it is a merely an old falsehood that wasn't scientifically debunked until laryngoscopes came into play.