# Ontology
## Overview
**Ontology** is the philosophical study of being — what exists, what kinds of things exist, and what it means to exist. In information science and computing, the word has been repurposed to mean a formal model of knowledge within a domain, defining concepts and their relationships.
## Etymology
The term was coined in Neo-Latin as *ontologia* by German philosopher Jacob Lorhard in 1606 (in his *Ogdoas Scholastica*), from Greek *on* (genitive *ontos*), 'being, that which is' — the present participle of *einai* ('to be') — plus *-logia* ('study of'). Greek *einai* descends from PIE **\*h₁es-** ('to be, to exist').
PIE **\*h₁es-** is perhaps the most fundamental reconstructed root — it encodes the concept of existence itself. Its reflexes appear in every major Indo-European branch:
- **English**: *is*, *am* (partially suppletive) - **German**: *ist* ('is') - **Latin**: *est* ('is'), *esse* ('to be'), *essentia* ('essence') - **French**: *est* ('is'), *être* ('to be') - **Greek**: *esti* ('is'), *einai* ('to be'), *ousia* ('being, substance') - **Sanskrit**: *asti* ('is'), *sat* ('being, truth') - **Russian**: *yest'* ('is') - **Hittite**: *eszi* ('is') — the oldest attested form
From Latin *esse* and its derivatives, English gets **essence** ('the being of something'), **essential**, **entity** (via *ens*, present participle of *esse*), **absent** (*ab-* + *esse*, 'being away'), **present** (*prae-* + *esse*, 'being before'), and **interest** (*inter* + *esse*, 'being between').
## Philosophical Ontology
Aristotle called the study of being *prōtē philosophia* ('first philosophy') — the investigation of 'being qua being' (*to on hē on*), meaning being considered simply as being, stripped of particular qualities. What does it mean for something to exist? What categories of existence are there? What is the relationship between substance and attributes?
Major ontological positions include:
- **Realism**: entities exist independently of our perception or concepts - **Idealism**: reality is fundamentally mental or idea-based - **Nominalism**: only particular things exist; universals are mere names - **Materialism**: only physical matter exists - **Dualism**: both mental and physical substances exist
## Computational Ontology
In computer science and information technology, an **ontology** is a formal specification of a domain's concepts, properties, and relationships. The Web Ontology Language (OWL) provides a standard for defining ontologies on the Semantic Web. Database schemas, knowledge graphs, and taxonomies are all forms of ontological modeling.
This appropriation of the philosophical term reflects a genuine conceptual parallel: both philosophical and computational ontology ask 'what kinds of things exist in this domain, and how are they related?'
## Related Forms
The family includes **ontological** (adjective), **ontologist** (noun), and compounds like **meta-ontology** (the study of ontology itself — what are we doing when we do ontology?). The **ontological argument** for God's existence, formulated by Anselm of Canterbury (1078), argues that the concept of a maximally great being entails that being's existence.