- A grouping of things based on shared common attributes.
Definition from: NCI Thesaurus via Unified Medical Language System at the National Library of Medicine
- A group, set, or kind marked by common attributes or a common attribute; especially : a major category in biological taxonomy ranking above the order and below the phylum or division (the class Mammalia).
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.