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