synonym noun [C]
a word or phrase which has the same or nearly the same meaning as another word or phrase in the same language
The words 'small' and 'little' are synonyms.
It is sometimes said that there are no true synonyms in the English language, and that all words have a slightly different meaning.
Compare antonym.

synonymous adjective
The words 'annoyed' and 'irritated' are more or less synonymous.
The word 'annoyed' is more or less synonymous with 'irritated'.

If you say that one thing is synonymous with another, you mean that the two things are so closely connected in most people's minds that one suggests the other.
'Chanel' is for most people synonymous with style.
Oscar Wilde's name is synonymous with wit.