Snow Owl is an authoring platform for clinical terminologies. Amongst its many useful features, you will find the support of Expression Constraint Language (ECL) for semantic querying, the ability to create intensional and extensional subsets among other reference set types that follow the SNOMED International (formerly known as IHTSDO) SNOMED CT RF2 specifications, as well as collaborative authoring. In addition to authoring SNOMED CT or a national extension, Snow Owl supports a variety of other ontologies and classifications. Snow Owl is designed to be a framework to support higher-level terminology authoring tools, such as drug and device dictionaries.