We found 16 dictionaries that define the word a horizon:

General (14 matching dictionaries)

1. A horizon: Merriam-Webster
2. A horizon: American Heritage Dictionary of the English Language
3. A horizon: Collins English Dictionary
4. A-horizon, A horizon: Vocabulary.com
5. A-horizon: Webster's New World College Dictionary, 4th Ed.
6. A horizon: The Wordsmyth English Dictionary-Thesaurus
7. A horizon: Infoplease Dictionary
8. A horizon, a-horizon: Dictionary.com
9. A horizon: Wikipedia, the Free Encyclopedia
10. A-horizon, A horizon: Rhymezone
11. a-horizon, a horizon: FreeDictionary.org
12. a-horizon, a horizon: Mnemonic Dictionary
13. A-horizon, A horizon: TheFreeDictionary.com
14. A horizon: Merriam-Webster

Science (1 matching dictionary)

15. Botanical Terms (No longer online)

Tech (1 matching dictionary)

16. A horizon: Canadian Soil Information System

(Note: See a_horizons as well.)

Definitions from WordNet (A horizon) ▸ noun:  the top layer of a soil profile; usually contains humus Opposite: zenith, peak, summit, apex ▸ Words similar to a horizon ▸ Usage examples for a horizon ▸ Idioms related to a horizon ▸ Wikipedia articles (New!) ▸ Words that often appear near a horizon ▸ Rhymes of a horizon ▸ Invented words related to a horizon