The correct answer is to find a synonym for a word that the writer has used repeatedly and wants to vary.
A thesaurus is a dictionary of synonyms - you would open up a thesaurus if you want to find words that have the same (or very similar) meanings as another word.
If you want to check pronunciation, you can look it up in a regular dictionary. If you want to see the history of a word, you would need an etymological dictionary.