We found 17 dictionaries that define the word 18:

General (12 matching dictionaries)

1. 18: Vocabulary.com
2. 18: Wordnik
3. 18: Cambridge Advanced Learner's Dictionary
4. 18: Dictionary.com
5. 18: Rhymezone
6. 18: FreeDictionary.org
7. 18: Mnemonic Dictionary
8. 18: TheFreeDictionary.com
9. 18: Longman Dictionary of Contemporary English
10. 18: Merriam-Webster

Art (1 matching dictionary)

11. 18: Literary Terms

Computing (1 matching dictionary)

12. 18, 18 (number): Encyclopedia

Miscellaneous (1 matching dictionary)

13. 18: Acronym Finder

Science (1 matching dictionary)

14. 18: Eric Weisstein's World of Mathematics

Tech (1 matching dictionary)

15. Basics of Space Flight Glossary (No longer online)

(Note: See eighteen as well.)

Definitions from WordNet (18) ▸ noun:  the cardinal number that is the sum of seventeen and one ▸ adjective:  being one more than seventeen Similar: 16, 19, cardinal, 21, xviii, adult, grant, hole, case, district, more... Opposite: young, juvenile, minor, immature, underage Types: eighteen, eighteen years old, XVIII, more... Phrases: atomic number 18, trisomy 18 syndrome, fludeoxyglucose f 18, 18 karat gold, 18 Wheeler, more... ▸ Words similar to 18 ▸ Usage examples for 18 ▸ Idioms related to 18 ▸ Wikipedia articles (New!) ▸ Popular adjectives describing 18 ▸ Popular nouns described by 18 ▸ Words that often appear near 18 ▸ Rhymes of 18 ▸ Invented words related to 18