The development of modern knowledge-intensive areas of human activity in the world is determined by the growing role of computer tech-nology. Today, the flow of information is growing significantly, and there is a need to find new ways of storing, presenting, formalizing, organizing it as well as automatic processing. Therefore, there is a growing interest in a comprehensive knowledge base that can be used for various practical pur-poses. There is a great need for systems based on neural networks that ex-tract any information from the text without the human factor. In the middle of the twentieth century, along with the World Wide Web, Semantic Web started to appear, which provided additional tags that carried information about the semantics of elements in hypertext pages. An integral part of the semantic web is the concept of ontology, which is a lexical database con-sisting of a network of words.This article analyzes the origin of the term ontology, the ontological views of philosophers, the thesaurus and the concepts of ontology. Factors of creating the thesaurus of information retrieval systems are covered.