2. Noun. the disappearance of the rhythmic peaks displayed on a heart monitor ¹
3. Verb. (intransitive) (context: of the heart) to stop beating ¹
4. Verb. (intransitive) (context: by extension) to die ¹
¹ Source: wiktionary.com
Definition of Flatline
1. to register as having no brain waves or heartbeat [v -LINED, -LINING, -LINES]