Minimal music is a style of music associated with the work of American composers La Monte Young, Terry Riley, Steve Reich, and Philip Glass. It originated in the New York Downtown scene of the 1960s and was initially viewed as a form of experimental music called the New York Hypnotic School. Prominent features of the style include consonant harmony, steady pulse (if not immobile drones), stasis or gradual transformation, and often reiteration of musical phrases or smaller units such as figures, motifs, and cells. It may include features such as additive process and phase shifting. Minimal compositions that rely heavily on process techniques that follow strict rules are usually described using the term process music.
Starting in the early 1960s as a scruffy underground scene in San Francisco alternative spaces and New York lofts, minimalism spread to become the most popular experimental music style of the late 20th century. The movement originally involved dozens of composers, although only five (Young, Riley, Reich, Glass, and later John Adams) emerged to become publicly associated with American minimal music. In Europe, the music of Louis Andriessen, Karel Goeyvaerts, Michael Nyman, Gavin Bryars, Steve Martland, Henryk Górecki, Arvo Pärt, and John Tavener exhibits minimalist traits.
It is unclear where the term minimal music originates. Steve Reich has suggested that it is attributable to Michael Nyman, a claim two scholars, Jonathan Bernard and Dan Warburton, have also made in writing. Philip Glass believes Tom Johnson coined the phrase.
Source: Wikipedia, "Minimal music", available under the CC-BY-SA License.