During last 10-15 years philosophy term ontology has rapidly come in the IT community. Its income is connected with many discussions and disputes about the usage of this term and correctness of its definition. The aim of this paper is to introduce to a reader the evolution of term’s definition to make it possible to her/him to choose more suitable and acceptable definition of ontology. There is also a variety in classifications of ontologies and almost each author describes his/her own classification scheme, which by point of view of the authors represents domain better and is more suitable for application. More then 10 different ontology definitions are analyzed, a historical evolution and fluidity of them are shown, as well as related comments. Ontology definitions are complemented with ontologies classification schemes. In the section 3 the author proposes to divide ontologies classification schemes by linking schemes into 4 groups according to their properties. The grouping of ontologies classification schemes proposed is based on analysis of 12 schemes, that allowed to make division into following 4 groups: ontology applying dependent classification schemes, ontology applying independent classification schemes, ontology representation structure-based classification schemes and concepts-based classification included in ontology schemes. The section 4 describes all analyzed schemes. The section 5 describes chances of merging ontologies classification schemes to get the specific schemes, which are needed for better and more expressive ontologies classification in the particular application.