Ontology is used with different meanings in different communities. Philosophically, ontology refers to the study of the nature and structure of reality, as defined by Aristotle. Computationally, it refers to a formal representation of shared conceptualizations, used to model the structure of systems. Computational ontologies are formal, explicit specifications of shared conceptualizations, allowing for the representation of entities, relations, and hierarchies. They are used to organize and represent knowledge in a structured way, enabling reasoning and inference. The concept of ontology has evolved from an early informal definition by Gruber in 1993, which described it as an "explicit specification of a conceptualization." In 1997, Borst defined it as a "formal specification of a shared conceptualization," emphasizing the need for shared understanding and formal representation. In 1998, Studer et al. merged these definitions, stating that an ontology is a formal, explicit specification of a shared conceptualization. This definition highlights the importance of shared explicit specifications in computational ontologies, which are essential for knowledge representation and integration. The paper discusses the distinction between philosophical and computational uses of ontology, emphasizing the importance of shared conceptualizations in computational contexts. It also outlines the structure of ontologies, including taxonomies, concepts, and relations, and their role in modeling systems and enabling reasoning.Ontology is used with different meanings in different communities. Philosophically, ontology refers to the study of the nature and structure of reality, as defined by Aristotle. Computationally, it refers to a formal representation of shared conceptualizations, used to model the structure of systems. Computational ontologies are formal, explicit specifications of shared conceptualizations, allowing for the representation of entities, relations, and hierarchies. They are used to organize and represent knowledge in a structured way, enabling reasoning and inference. The concept of ontology has evolved from an early informal definition by Gruber in 1993, which described it as an "explicit specification of a conceptualization." In 1997, Borst defined it as a "formal specification of a shared conceptualization," emphasizing the need for shared understanding and formal representation. In 1998, Studer et al. merged these definitions, stating that an ontology is a formal, explicit specification of a shared conceptualization. This definition highlights the importance of shared explicit specifications in computational ontologies, which are essential for knowledge representation and integration. The paper discusses the distinction between philosophical and computational uses of ontology, emphasizing the importance of shared conceptualizations in computational contexts. It also outlines the structure of ontologies, including taxonomies, concepts, and relations, and their role in modeling systems and enabling reasoning.