Thelema means “will” in Greek.
It’s the principle behind the magick systems of Aleister Crowley
Book of Law, Chapter 1, Verse 40:
40. Who calls us Thelemites will do no wrong, if he look but close into the word. For there are therein Three Grades, the Hermit, and the Lover, and the man of Earth. Do what thou wilt shall be the whole of the Law.
Links
wikipedia: thelema