Rock music with overtones of psychedelia. Generally blues based, like most rock, but often the most psychedelic aspects of this kind of music come from the juxtaposition of traditional rock sounds with outlandish ones (ie: When the Beatles used a sitar rather than a guitar).
The Psychedelic aspect can also come in the lyrics, though it is rare to see psychedelic lyrics without music that fits the part.
Popular techniques in this genre are to use exotic musical styles and warbly effects in order to simulate (or perhaps even STIMULATE) the effects of psychedelic drugs.
The definition of psychedelic rock is not set in stone, and therefore almost anything, even if it doesn't fit these criteria, can be included as long as it has that 'trippy' feel.
Mark: Would you like to hear some Psychedelic Rock?
Mandy: Psycedelic Rock sounds blue.. no, purple. Woah.
Taking Drugs to make music to take drugs to :D
Jim: wht the hell is psychedelic rock?
Hendrix: Taking Drugs to make music to take drugs to ;)