A synonym is a word (or a phrase) which means the same thing as another word or phrase.
Examples:
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.
One can find sets of synonyms in a thesaurus, which is a book listing groups of related words.