Merriam-Webster Collegiate Dictionary - synonym

 noun  Etymology: Middle English sinonyme, from Latin ~um, from Greek synonymon, from neuter of synonymos ~ous, from syn- + onyma name — more at name  Date: 15th century  1. one of two or more words or expressions of the same language that have the same or nearly the same meaning in some or all senses  2.  a. a word or phrase that by association is held to embody something (as a concept or quality) a tyrant whose name has become a ~ for oppression  b. metonym  3. one of two or more scientific names used to designate the same taxonomic group — compare homonym  • ~ic also ~ical adjective  • ~ity noun