Delta wave
A delta wave is a brain wave and is usually associated with deep sleep or coma.
When Dr. Julian Bashir was attacked by a Lethean called Altovar in 2371 he fell in a coma. He found himself on DS9 after the attack. However he learned that the station represented his mind, since the tricorder was only able to pick up delta waves from Dr. Bashir and nothing else. (DS9:Distant Voices)