delta wave

This term is used primarily in clinical neurology and sleep medicine to describe the slowest of the standard brain wave patterns. It carries a strong association with deep sleep, specifically stage 3 non-REM sleep, and is often used as a biomarker for the depth of unconsciousness or certain pathological states when appearing during wakefulness.

In a medical context, the term is typically used in the plural to describe a pattern of activity on an electroencephalogram (EEG). While it refers to a physical electrical oscillation, it is treated as a discrete countable entity in clinical reporting.