Question

A thesaurus is primarily used to look up

Answer 1

the synonyms and antonyms of words. Antonyms are words that have opposite meanings. Synonyms are words that have the equivalent meaning or relatively the same. The Homonyms are words that are spelled and said the same but have dissimilar meanings

Answer 2

A thesaurus is primarily used to look up synonyms.
A thesaurus is a dictionary which is special in that it only lists synonyms of words. A synonym is a word that has the same (or close) meaning to another word. So, for example, if you wanted to see what the synonyms for the word 'happiness' are, you would search for them in a thesaurus, and you would find: contentment, pleasure, satisfaction, glee, mirth, etc.