A delta wave is a high amplitude

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

 brain wave with a frequency of oscillation between 0–4 hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

. Delta waves, like other brain waves, are recorded with an electroencephalogram

Electroencephalography

Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...

 (EEG) and are usually associated with the deepest stages of sleep (3 and 4 NREM), also known as slow-wave sleep

Slow-wave sleep

Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...

 (SWS), and aid in characterizing the depth of sleep.

"Delta waves" were first described in the early 1900s by W. Grey Walter, who improved upon Dr.