The need of a common ontology for describing orthology information in biological research communities has led to the creation of the Orthology Ontology (ORTH). ORTH ontology is designed to describe sequence homology data available in multiple orthology databases on the Web (e.g.: OMA, OrthoDB, HieranoiDB, and etc.). By sequence homology data, we mostly mean gene region, gene and protein centric orthology, paralogy, and xenology information. Depending on the database, the homology information is structured in different ways. ORTH ontology accommodates these disparate data structures namely Hierarchical Orthologous Group (HOG), cluster of homologous sequences and homologous-pairwise relations between sequences. In addition to the specific ORTH terms, this specification includes terms of the imported ontologies (e.g. Semanticscience Integrated Ontology, SIO) which are pertinents to represent the information from various orthology databases in a homogeneous way.