Synonyms are words with similar meanings. They are tools used to replace other words in a sentence. The part of speech of a synonym will depend on the word it is replacing.
Here's an example:
* Word: Happy
* Synonym: Joyful
* Part of Speech: Both are adjectives.
The synonym "joyful" is an adjective because it replaces the adjective "happy."
So, while synonyms can be used in any part of speech, they are not themselves categorized as nouns, verbs, adverbs, or adjectives.