Dictionaries help decode words, supply definitions and indicate correct spelling.

Different dictionaries serve different purposes, with varying levels of information and quality. Some of the most highly recommended monolingual dictionaries for English are:

• Webster’s
• American Heritage Dictionary
• Dictionary.com
• Oxford English Dictionary, or the OED, includes the history of words and sentences in which they have been used across time.
• Other reputable publishers of dictionaries are the Cambridge and Collins dictionaries. See also: YourDictionary.com.

Another resource that offers spelling tips, exercises, thesauri, an open dictionary, and a pronunciation assistance for dictionary entries is:

MacMillan Dictionary

OneLook is a metadictionary site that finds words even when you search on a partial spelling. The site uses a simple search box and provides an uncluttered interface.

OneLook Dictionary

Thesauri (singular: thesaurus) assist writers with varying words and phrases by providing synonyms and antonyms.

English-language thesauri come in two basic formats: a newer dictionary format, and Roget’s original structure. The former, as you might guess, has entries arranged alphabetically with synonyms and antonyms at each main entry (accessed via an index). The latter is essentially a set of semantic fields, subdivided by topic, organized within each topic by part of speech, all accessed by a very large index at the end.

Be careful with thesauri, if you are unsure of the denotation (reference) and connotation (its implications beyond its direct reference), use a thesaurus with other dictionaries.

NOTE: Avoid varying a term too much because readers need to track the ideas or concepts throughout the work. For example, if a paper references a particular dataset, a writer should use the term dataset throughout the work to ensure consistency. However, if one uses a word such as “good” or “strong” throughout a work, the variety in those descriptive words give the reader a more robust document to assess.

In addition to some of the previously mentioned resources, here are two more thesauri:

The Oxford Paperback (Online) Thesaurus

and

Thesaurus.com, which is search only.

Usage Manuals

These guides contain information regarding grammar and word choice, particularly problematic expressions (e.g., “effect” vs. “affect”):

• Garner’s Modern American Usage (2009), 3rd ed.–a standard reference work for copy editors. Garner is also well-known for his Redbook, a legal style guide and other legal reference works.
• Fowler’s Dictionary of Modern English Usage (2015), 4th ed., online
• Strunk and White’s Elements of Style (2009), 4th ed, special reprint
• Williams and Colomb’s Style: Lessons in Clarity and Grace (2014), 11th ed.
• Grammar Girl–Quick tips for learning and remembering lessons such as can vs. may, effect vs. affect, and the spelling of the color gray/grey