Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin but well-structured atmosphere. Triton orbits Neptune in a retrograde orbit—an orbit in the direction opposite to its planet's rotation—the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet, captured from the Kuiper belt into Neptune orbit.
Reconstructed enhanced color image using imagery from Voyager 2, August 1989. Triton's south polar cap dominates most of the image.
William Lassell, the discoverer of Triton
Triton (lower left) compared to the Moon (upper left) and Earth (right), to scale
Clouds observed above Triton's limb by Voyager 2.
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.
Size comparison of Earth and the Moon
Artist impression of Rhea's proposed rings