EEG activity with a frequency of less than 4 Hz (usually 0.1-3.5 Hz). In human sleep scoring, the minimum characteristics for scoring delta waves are conventionally 75 µV (peak to peak) amplitude, and 0.5-second duration (2 Hz) or less.*

*Reprinted with permission from: Culebras, Antonio Clinical Handbook of Sleep Disorders Hardcover © 1996 Butterworth-Heinemann