A synonym is a word (or sometimes a phrase) which means the same thing as another word or phrase.
Synonyms can be nouns, verbs, adverbs or adjectives, but both words must be of the same part of speech. That means, only a noun can be a synonym of another noun, only a verb can be a synonym of another verb, and so on.
The word "synonym" dates back over 500 years, to late Middle English. The term is derived from Latin from the Greek word sunōnumon, neuter form (used as a noun) of the adjective sunōnumos, from sun- meaning 'with' + onoma meaning 'name' in the Greek language.
The meaning of the word has remained unchanged for all these centuries. There is even a saying, going back to 1892, "Once a synonym, always a synonym". The word has been taught to generations of English-language students and is commonly known by the general public. Many other languages have a similar word for "synonym" with the same or similar spelling.
Synonym Facts for Kids. Kiddle Encyclopedia.