intransitive verb. To show a horizontal line on a monitor indicating no electrical activity in an electrocardiogram or an electroencephalogram.

intransitive verb. To die.

intransitive verb. To be in an unchanging condition.

adjective. Being or relating to an electrocardiogram or an electroencephalogram that demonstrates no electrical activity.

noun. asystole; the absence of heart contractions

noun. the disappearance of the rhythmic peaks displayed on a heart monitor

verb. (of the heart) to stop beating

verb. (by extension) to die

verb. To stay at the same level, without development; or, to fall.

"The beeping the pilot heard was "flatline" - the long continously beep meaning "you are about to die unless you deploy your canopy NOW" it's the same sound as in heart monitors), which apparently caused him to stop the fast dive, since canopy deployment was logged at 1500 feet :-)"