Full Idea
The word 'ontology' is derived from the Greek word 'ontia', which means 'things which exist'.
Clarification
Ontology is perhaps the main branch of traditional (and much modern) metaphysics
Gist of Idea
'Ontology' means 'study of things which exist'
Source
Keith T. Maslin (Introduction to the Philosophy of Mind [2001], 1.1)
Book Reference
Maslin,Keith: 'An Introduction to the Philosophy of Mind' [Polity 2001], p.3