A desk dictionary and thesaurus are essential tools for choosing precise words. The dictionary gives a word's denotations (its exact definitions) and its etymology (its family history). In checking a definition in the dictionary, pay attention to the parts of speech, the different functions a word can have in a sentence. The same word can have a different meaning when its grammatical function changes. The following links to online dictionaries you might find helpful in the course of writing your essay:
www.freesearch.co.uk/dictionary
A thesaurus provides synonyms, words with the same or nearly the same meaning. Just as siblings, even identical twins, differ subtly from each other, so do words differ subtly from their synonyms. Because words have different connotations, that is, secondary or suggested meanings, some synonyms work in one context but not in another. Synonyms can have either positive or negative connotations. For example, in describing someone's manner of speaking, the word direct is usually positive, but blunt is not. The following link to some useful online thesauri:
Please note: for writing in Canada, either in class or at work, you should also consult specifically Canadian dictionaries and thesauri, as both spelling and usage can differ in Canadian English.