- The theory that the sun is the center of the universe, (This theory is historically important and was widely accepted at the time of Copernicus, Galileo and Kepler.)
- (astronomy) The theory of the heliocentric model, that the planets including Earth orbit the Sun, in contrast to geocentrism. (This theory, that the sun is the centre of the Solar System only is the current theory.)
Many dictionaries recognize "heliocentricism" as the grammatically correct form of this word. However, in current usage heliocentrism appears to be by far more frequently used. According to a Google Books search it seems that heliocentrism began to gain ground in the turn of 1960's and 1970's with the first occurrence taking place in 1930's.