Definition from Wiktionary, the free dictionary
- Worship of Satan (usually synonymous with "the Devil").
- Any religion seen as either non-Christian or anti-Christian.
- A profession, philosophy, or ideological construct featuring a positive, admirable, or useful association to the figure, character or entity known as 'Satan'.
- The religion of the Church of Satan, founded in 1966 by Anton Szandor LaVey, which characterizes human instincts and desires as "Satan" and urges capitulation to such instincts and desires; also called LaVeyan Satanism.
Usage notes 
- Satanism is not always capitalized in all senses.
- Self-identified Satanists typically use the term Satanism in senses 3 and 4, and use the terms Satanic and Satanist analogously.
a religion founded by Anton Szandor LaVey