Heliocentrism is the idea that the sun is the center of the universe; it is opposed to the Ptolemaic system and other geocentric systems, according to which Earth is at the center of the universe. More broadly, heliocentrism refers to the idea that the planets are orbiting Sol; heliocentrists soon recognized (by Galileo's time) the possibility that the sun is just a star in the Milky Way, and in the 20th century it became clear that the latter is just one of the galaxies in the universe.

Heliocentric comes from the Greek, "Helios", for Sun. The heliocentric model of the solar system, in which it is argued that Earth orbits Sol, was first proposed by Aristarchus (c.270 BC). The theory was revived in modern times by Nicolaus Copernicus (q.v.), in a form consistent with then-current observations. This theory solved the issue of planetary retrograde motion (with respect to the celestial sphere), by arguing that such motion was only perceived and apparent, rather than real. This issue was also resolved in the geocentric Tychonian system; the latter, however, while eliminating the major epicycles, retained as a physical reality the irregular back-and-forth motion of the planets, which Kepler characterized as a "pretzel". Heliocentrism was notably advanced by Galileo, Kepler, and Newton.

Heliocentric theory was famously resisted by the Catholic church, which taught the Ptolemaic system—that the Earth was the center of the universe and all celestial bodies orbited it— and was also tolerant of the Tychonian system because it was geocentric. For advancing heliocentric theory Galileo was famously put under house arrest for the last several years of his life. The Catholic support for geocentricism should not be confused with the idea of a flat earth, which the Church never supported.