Semantic similarity measures quantify how ``alike'' (or similar) two concepts are by determining their closeness in a hierarchy. Semantic relatedness measures quantify degree of semantic feature overlap between two terms. Two terms may be related (needle-thread) but not necessarily similar.
In simple terms, a Concept Unique Identifier (CUI) is a concept in the UMLS Metathesaurus. For a longer explanation, read on: The UMLS Metathesaurus contains biomedical and clinical concepts from over 100 disparate terminology sources that have been semi- automatically integrated into a single resource containing a wide range of biomedical and clinical information. For example, it contains the Systematized Nomenclature of Medicine--Clinical Terms (SNOMED CT), which is a comprehensive clinical terminology created for the electronic exchange of clinical health information, the Foundational Model of Anatomy (FMA), which is an ontology of anatomical concepts created specifically for biomedical and clinical research, and MEDLINEPLUS, which is a terminology source containing health related concepts created specifically for consumers of health services. The concepts in these sources can overlap. For example, the concept Autonomic nerve exists in both SNOMED CT and FMA. The Metathesaurus assigns the synonymous concepts from the various sources a CUI. Thus both the Autonomic nerve concepts in SNOMED CT and FMA are assigned the same CUI (C0206250). This allows multiple sources in the Metathesaurus to be treated as a single resource.