1) Semantic relation 2) Synonymousness 3) Synonymy 1) Synonymousness 2) Synonymy 
synonymy, synonymity 1. The state of being a synonym. 2. The semantic relation that holds between two words which can (in a given context) express the same meaning. 3. The study, classification, and distinguishing of synonyms. 4. The relation between two words or phrases with the same meaning; such as, 'gift' and 'donation'. ...