In planetary nomenclature, a tholus is a small domical mountain or hill. The word is from the Greek θόλος, tholos, which means a circular building with a conical or vaulted roof. The Romans transliterated the word into the Latin tholus, which means cupola or dome. In 1973, the International Astronomical Union (IAU) adopted tholus as one of a number of official descriptor terms for topographic features on Mars and other planets and satellites. One justification for using neutral Latin or Greek descriptors was that it allowed features to be named and described before their geology or geomorphology could be determined. For example, many tholi appear to be volcanic in origin, but the term does not imply a specific geologic origin. Currently, the IAU recognizes 56 descriptor terms. Tholi are present on Venus, Mars, asteroid 4 Vesta, dwarf planet Ceres, and on Jupiter's moon Io.

Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can be easily located, described, and discussed. Since the invention of the telescope, astronomers have given names to the surface features they have discerned, especially on the Moon and Mars. To found an authority on planetary nomenclature, the International Astronomical Union (IAU) was organized in 1919 to designate and standardize names for features on Solar System bodies.