synonymy

1. the fact of two or more words or expressions having the same meaning
   Word Origin
2. quality of being similar in meaning; study of synonyms; list of words with similar or related meanings;
3. the semantic relation that holds between two words that can (in a given context) express the same meaning
4. A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words begin, start, commence, and initiate are all synonyms of one another: they are synonymous .