Psychedelia in music (or also psychedelic music, less formally) is a term that refers to a broad set of popular music styles, genres and scenes, that may include psychedelic rock, psychedelic folk, psychedelic pop, psychedelic soul, psychedelic ambient, psychedelic trance, psychedelic techno, and others. Psychedelic rock is also commonly called acid rock.

During the mid to late 1960s music of the psychedelic genre, in the basic sense of the word, was incorporated by almost every band/recording artist at that time (which makes psychedelia a very broad style). However, psychedelia as a strict genre of music is difficult to identify/label because of the many contrasting musical groups that interpreted then demonstrated this concept as their own form of music. It is easy to identify/stereotype music as being "psychedelic" by its potential use of elaborate studio effects (heavy reverb), exotic instrumentation (sitar), and surreal lyrics, but to strictly apply these and other elements as a basis for musical classification is almost irrelevant. Even the heavier and more aggressive bands of the late 60's (MC5, The Litter, Iron Butterfly), implemented typical psychedelic techniques into their songs, but psychedelia was also strongly associated with the lighter or sometimes more mainstream acts of the era (The Beatles, Strawberry Alarm Clock, Sopwith Camel).

Source : http://en.wikipedia.org/wiki/Psychedelia