/ Dictionary / Index T theta rhythm: Meaning and Definition of Find definitions for: the'ta rhythm" Pronunciation: [key] — Physiol. Physiol. a pattern of brain waves having a regular frequency of 4 to 7 cycles per second as recorded by an electroencephalograph, observed during various states of light sleep or arousal. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. theta theta wave Related Content Daily Word Quiz: frugal Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language