Terence Michael Joseph "Geezer" Butler (born 17 July 1949 in Aston, Birmingham, England) is the founding bassist for heavy metal legends Black Sabbath. His nickname supposedly arises from his habit as a teenager of referring to everyone as "Geezer".

Geezer formed his first band, Rare Breed, with schoolmate John "Ozzy" Osbourne in the autumn of 1967. Separated for a time, Ozzy and Geezer reunited in the blues foursome, Polka Tulk, along with Guitarist Tony Iommi and drummer Bill Ward. They renamed their band Earth, but after finding a band in the small-time English circuit with the same name, soon adopted Black Sabbath in early 1969.

Butler played rhythm guitar in his pre-Sabbath days, including with Rare Breed, but when Sabbath was formed, Iommi made it clear that he wouldn't want to play with another guitarist, so Geezer moved to bass.

Geezer is noted as being one of the first bassists to use a Wah-wah pedal on his bass, as showcased at the beginning of N.I.B. which inspired many later bassists, such as Cliff Burton who cited him as an influence. He was also one of the first bassists to de-tune, the reason being so he could play Iommi's riffs with more ease after he had de-tuned down to C# (one and a half tones down) himself. De-tuning was soon adopted as a standard in most heavy metal bands to follow.

The name "Black Sabbath" was borrowed from a song written by Butler, who got the name from the Boris Karloff film of the same name, at the time he was an avid follower of occultic novelist Dennis Wheatley. Although Ozzy was the focal point of the band, Butler wrote most of the band's lyrics, drawing heavily upon his fascination with the black arts to explore recurring themes of death and destruction.

During the latter half of the 1970s, Black Sabbath's popularity dwindled, although the group continued on in the early 1980s with ex-Rainbow frontman Ronnie James Dio and then with ex-Deep Purple frontman Ian Gillan. Butler quit the band in the middle of 1984, forming the Geezer Butler Band. In 1988 he joined Ozzy Osbourne to take place in the No Rest For The Wicked World Tour. Butler re-joined Black Sabbath in 1991 for the reunion of the "Mob Rules" lineup, but again quit the group after the Cross Purposes tour of 1994.

By 1995, Geezer was back together with Osbourne, to play on the Ozzmosis album. After recording Ozzmosis, he formed G/Z/R, issuing "Plastic Planet" in 1995. His next solo album, "Black Science" followed in 1997. Geezer returned to Sabbath one more time for the 1997 edition of Ozzfest, and has remained with the band since. In 2005, he released Ohmwork, his third solo album.