A heliocentric system is a model of the universe that assumes heliocentricity (Sun-centered, or heliocentric), i.e., that the planets orbit the Sun. This type of model displaced Earth-centered systems (geocentric systems such as the ancient homocentric-sphere models that explained motions in the sky using the rotations of nested interconnected invisible spheres centered on Earth), i.e., which suppose planets orbit the Earth, in part, by offering a more straight-forward means of calculating the apparent motion of planets in the celestial sphere.
Given the heliocentric system, it was clear some planets were closer to the Sun than Earth (called inferior planets) and some further (superior planets). The inferior planets showed a maximum angle from the Sun (elongation) and the superior planets would reach an angle of 180° from the Sun (conjunction).
Current usage of the terms heliocentric system and heliocentrism (acceptance of such a system) is not consistent regarding whether the terms imply the Sun is the center of the entire universe, versus merely the claim that the planets orbit around it. Historically, both Sun-centered models and models in which stars were "distant suns" were proposed by ancient Greeks, though neither was favored at the time. The precise meaning of heliocentric system in this respect (whether it is meant to apply to the whole universe) seems merely of historical interest.