[from cyberpunk SF, refers to flattening of EEG traces upon brain-death] (also adjectival flatlined).
1. To die, terminate, or fail, esp. irreversibly. In hacker parlance, this is used of machines only, human death being considered somewhat too serious a matter to employ jargon-jokes about.
2. To go completely quiescent; said of machines undergoing controlled shutdown. “You can suffer file damage if you shut down Unix but power off before the system has gone flatline.”
3. Of a video tube, to fail by losing vertical scan, so all one sees is a bright horizontal line bisecting the screen.