theta wave [Gk, theta, eighth letter of Greek alphabet; AS, wafian] , one of the several types of brain waves, characterized by a relatively low frequency of 4 to 7 Hz and a low amplitude of 10 μV. Theta waves are the “drowsy waves” of the temporal lobes of the brain and appear in electroencephalograms when the individual is awake but relaxed and sleepy. Also called theta rhythm. Compare alpha wave, beta wave, delta wave.
