When two or more words have the same or nearly the same meaning, they are synonyms. Some synonyms are exactly the same in certain senses—for example, student and pupil, buy and purchase, stroll and saunter, rush and hurry, language and tongue, and on and upon. Others aren’t exactly alike, but close enough to be called synonyms—for example, walk and stroll, job and gig, talk and lecture, teenager and youth, brown and tawny, and ocean and sea.
Although we usually think of synonyms as individual words, phrases can also be synonyms. For example, motor vehicle and automobile are synonymous.
The noun synonym makes synonymous (adjective) and synonymously (adverb). The opposite of synonym is antonym—that is, a word whose meaning is the opposite of that of another word. For example, synonym and antonym are antonyms. Antonym and opposite are synonyms.