A delta wave is a brain pattern usually associated with deep sleep or a coma.
When Doctor Julian Bashir was attacked by a Lethean named Altovar in 2371 he fell in a coma. He found himself on Deep Space 9 after the attack, however he learned that the station represented his mind, since the tricorder was only able to pick up delta waves from himself and nothing else. (DS9: "Distant Voices")