delta wave, 1. the slowest of several types of brain waves, characterized by a frequency of 4 Hz and a relatively high voltage. Delta waves are “deep-sleep waves” associated with a dreamless state from which an individual is not easily aroused. Also called delta rhythm. Compare alpha wave, beta wave, theta wave. 2. (in cardiology) a slurring of the QRS portion of an electrocardiogram tracing caused by preexcitation in Wolff-Parkinson-White syndrome.
