本体论（英语：Ontology），又译存在论、存有论，它是形而上学的一个基本分支，本体论主要探讨存有本身，即一切现实事物的基本特征。

有的哲学家，如柏拉图学派认为：任何一个名词都对应着一个实际存在；另外一些哲学家则主张有一些名词并不代表存在的实体，而只代表一种集合的概念，包括事物或事件，也有抽象的，由人类思维产生的事物。例如“社团”就代表一群具有同一性质的人组成的集合；“几何”就代表一种特殊知识的集合等。

本体论就是「研究到底哪些名词代表真实的存在实体，哪些名词只是代表一种概念」。所以本体论成为某些哲学分支的基础。近年来，人工智能及信息技术相关领域的学者也开始将本体论的观念用在知识表达上，即借由本体论中的基本元素：概念及概念间的关连，作为描述真实世界的知识模型。针对此一趋势，W3C组织也开始定义了许多本体论的相关语言，如RDF、DAML+OIL、OWL等。

Ontology is the philosophical study of the nature of being, becoming, existence, or reality, as well as the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology often deals with questions concerning what entities exist or may be said to exist, and how such entities may be grouped, related within a hierarchy, and subdivided according to similarities and differences. Although ontology as a philosophical enterprise is highly theoretical, it also has practical application in information science and technology, such as ontology engineering.