Delta wave

A delta wave is a brain pattern which is usually associated with deep sleep or coma.

Upon examining Simon Van Gelder aboard the USS Enterprise (NCC-1701) in 2266, Doctor Leonard McCoy's electroencephalagraph dedicated bursts of delta waves in Van Gelder's brain which despite being highly abnormal, didn't indicate schizophrenia, tissue damage or any other condition with which McCoy was familiar with. The Enterprise crew later discovered that it was caused by Dr. Tristan Adams' neural neutralizer. 

Following an attack by the Lethean Altovar in 2371, Dr. Julian Bashir fell into a coma. While in a coma, he found himself on Deep Space 9 after the attack, however he learned that the station only represented his mind because his tricorder was only able to detect his delta waves.