Definition of delta wave
: a high amplitude electrical rhythm of the brain with a low frequency of less than four cycles per second that occurs especially in slow-wave sleep, is most prominent in infancy and early childhood, and may exhibit abnormal activity in various conditions (such as traumatic brain injury or dementia)
— called also delta, delta rhythm
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Merriam-Webster unabridged
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Time Traveler for delta wave
The first known use of delta wave was in 1936
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