- to die or be so near death that the display of one's vital signs on medical monitoring equipment shows a flat line rather than peaks and troughs
- to remain at a continuous low level
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for flatline
by 1998, from the flat line on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.
Online Etymology Dictionary, © 2010 Douglas Harper