Michael Servetus (; Spanish: Miguel Serveto as real name; French: Michel Servet; also known as Miguel Servet, Miguel de Villanueva, Revs, or Michel de Villeneuve; 29 September 1509 or 1511 27 October 1553) was a Spanish theologian, physician, cartographer, and Renaissance humanist. He was the first European to correctly describe the function of pulmonary circulation, as discussed in Christianismi Restitutio (1553). He was a polymath versed in many sciences: mathematics, astronomy and meteorology, geography, human anatomy, medicine and pharmacology, as well as jurisprudence, translation, poetry, and the scholarly study of the Bible in its original languages.

He is renowned in the history of several of these fields, particularly medicine. He participated in the Protestant Reformation, and later rejected the Trinity doctrine and mainstream Catholic Christology. After being condemned by Catholic authorities in France, he fled to Calvinist Geneva where he was denounced by John Calvin himself and burned at the stake for heresy by order of the city's governing council.

Heresy is any belief or theory that is strongly at variance with established beliefs or customs, in particular the accepted beliefs of a church or religious organization. The term is usually used in reference to violations of important religious teachings, but is also used of views strongly opposed to any generally accepted ideas. A heretic is a proponent of heresy.The term is used particularly in reference to Christianity, Judaism, and Islam. In certain historical Christian, Muslim, and Jewish cultures, among others, espousing ideas deemed heretical has been (and in some cases still is) met with censure ranging from excommunication to the death penalty.

Heresy is distinct from apostasy, which is the explicit renunciation of one's religion, principles or cause; and from blasphemy, which is an impious utterance or action concerning God or sacred things. Heresiology is the study of heresy.