We found 21 dictionaries that define the word photoperiod:

General (11 matching dictionaries)

1. photoperiod: Merriam-Webster
2. photoperiod: Oxford Learner's Dictionaries
3. photoperiod: American Heritage Dictionary of the English Language
4. photoperiod: Collins English Dictionary
5. photoperiod: Wordnik
6. photoperiod: Wiktionary
7. photoperiod: Webster's New World College Dictionary, 4th Ed.
8. photoperiod: Infoplease Dictionary
9. photoperiod: Dictionary.com
10. Photoperiod: Wikipedia, the Free Encyclopedia
11. photoperiod: TheFreeDictionary.com

Computing (1 matching dictionary)

12. photoperiod: Encyclopedia

Medicine (6 matching dictionaries)

13. photoperiod: Merriam-Webster Medical Dictionary
14. Dorland's Illustrated Medical Dictionary (No longer online)
15. Medical Dictionary (No longer online)
16. online medical dictionary (No longer online)
17. Sleep Terms (No longer online)
18. photoperiod: Medical dictionary

Science (1 matching dictionary)

19. Botanical Terms (No longer online)

Tech (2 matching dictionaries)

20. Dairy Glossary (No longer online)
21. Glossary of Meteorology (No longer online)

(Note: See photoperiodic as well.)

Definitions from Wiktionary (photoperiod) ▸ noun:  (biology) The normal duration of natural daylight experienced by an organism; daylength Similar: daylength, photoperiodism, photoperiodicity, scotoperiod, thermoperiod, photoregime, bioperiodicity, ahemeral day, photophil, photoentrainment, more... Opposite: scotoperiod, dark period, night period Adjectives: short, longest, critical, hour, daily, natural, day, dark, constant, artificial, normal ▸ Words similar to photoperiod ▸ Usage examples for photoperiod ▸ Idioms related to photoperiod ▸ Wikipedia articles (New!) ▸ Popular adjectives describing photoperiod ▸ Words that often appear near photoperiod ▸ Rhymes of photoperiod ▸ Invented words related to photoperiod