- 1[intransitive] to die
- 2[intransitive] to be at a low level and fail to improve or increase He said the party had been “flatlining” — failing to increase its level of support — for eight years.
NAmE//ˈflætlaɪn//(informal)Verb Forms present simple I / you / we / they flatline
he / she / it flatlines
past simple flatlined
-ing form flatlining