Ontology design is the systematic process and methodology for constructing formal conceptualizations of a domain. It involves identifying, defining, and structuring key concepts, properties, and relationships to create explicit, shareable, and machine-interpretable knowledge models. As a research field and methodological approach, it investigates principles, tools, and best practices for building high-quality ontologies, crucial for enabling semantic interoperability, automated reasoning, and robust knowledge representation in various computational systems.