 A number of philosophical schools in different parts of the ancient world held that the universe is composed of some kind of ‘atoms’ or minimal parts, albeit for different reasons. Although the modern term ‘atom’ derives from the ancient Greek adjective atomos, which literally means ‘uncuttable’, it is possible that the first theories we can describe as atomist were developed in classical Indian philosophy. While the ancient Greeks from whom we derive the modern term were theorizing about the fundamental constituents of the natural world, philosophical discussions have embraced various forms of minima, indivisible units or building blocks. Scholarly uses of the term ‘atomism’ range over mereological theories in widely different contexts (Zilioli 2021); not all possible senses of the term are discussed here. Several ancient atomist theories are treated in more depth in other articles in this encyclopedia: the reader is referred to individual entries for detailed discussions by the relevant experts (see the list of related entries at the end of this article).