Concepts are further described by various clinical terms or phrases, called Descriptions, which are divided into "Fully Specified Names" (FSNs), "Preferred Terms" (PTs), and "Synonyms". Each Concept has exactly one FSN, which is unique across all of SNOMED CT. It has, in addition, exactly one PT, which has been decided by a group of clinicians to be the most common way of expressing the meaning of the concept. It may have zero to many Synonyms. Synonyms are additional terms and phrases used to refer to this concept. They do not have to be unique or unambiguous.