Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Relating to or exhibiting periodic physiological activity that occurs more than once every 24 hours.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Cyclical with a periodicity significantly shorter than 24 hours, that is, more frequent than circadian.

Etymologies

ultra- + (circa)dian.
(American Heritage® Dictionary of the English Language, Fourth Edition)

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