We found 28 dictionaries that define the word letters:

General (16 matching dictionaries)

1. letters: Merriam-Webster
2. letters: Oxford Learner's Dictionaries
3. letters: Collins English Dictionary
4. letters: Vocabulary.com
5. Letter's, Letters, letter's, letters: Wordnik
6. letters: Cambridge Advanced Learner's Dictionary
7. letters: Wiktionary
8. letters: Dictionary.com
9. letters: Cambridge Essential American English Dictionary
10. LETTERS (novel), LETTERS, Letters (Butch Walker album), Letters (Jimmy Webb album), Letters (Matt Cardle album), Letters (song), Letters, The Letters (film): Wikipedia, the Free Encyclopedia
11. letters: Rhymezone
12. Letters: Dictionary of Phrase and Fable (1898)
13. letters: FreeDictionary.org
14. letters: Mnemonic Dictionary
15. letters: TheFreeDictionary.com
16. letters: Wikimedia Commons US English Pronunciations

Art (3 matching dictionaries)

17. Letters: Glossary of English Grammar Terms
18. The Organon: A Conceptually Indexed Dictionary (by Genus and Differentia) (No longer online)
19. ODLIS: Online Dictionary of Library and Information Science (No longer online)

Business (4 matching dictionaries)

20. letters: Merriam-Webster Legal Dictionary
21. letters: Law.com Dictionary
22. Everybody's Legal Dictionary (No longer online)
23. letters: Legal dictionary

Computing (1 matching dictionary)

24. letters: Encyclopedia

Medicine (1 matching dictionary)

25. letters: Medical dictionary

Miscellaneous (2 matching dictionaries)

26. Brilliant Dream Dictionary (No longer online)
27. letters: Idioms

Slang (1 matching dictionary)

28. Letters, the letters: Urban Dictionary

(Note: See letter as well.)

Definitions from Wiktionary (letters) ▸ noun:  Literature (school subject). ▸ noun:  The liberal arts, humanities, learning (broad accumulated cultural knowledge). Similar: missive, varsity letter, alphabetic character, letter of the alphabet, words, telegram, correspondence, alphabet, email, cablegram, more... Opposite: numbers, digits, figures, numerals, values Types: capital, lowercase, block, script, cursive, printed, handwritten, formal, more... Phrases: capital letters, letters patent, love letters, lowercase letters, greek letters, more... Adjectives: many, several, few, large, private, long, last, initial, personal, greek, original Colors:     red,     blue,     green,     yellow,     black, more... ▸ Words similar to letters ▸ Usage examples for letters ▸ Idioms related to letters ▸ Wikipedia articles (New!) ▸ Popular adjectives describing letters ▸ Words that often appear near letters ▸ Rhymes of letters ▸ Invented words related to letters