A flatline is an electrical time sequence measurement that shows no activity and therefore when represented, shows a flat line instead of a moving one. It almost always refers to either a flatlined electrocardiogram

Electrocardiogram

Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

, where the heart shows no electrical activity (asystole

Asystole

In medicine, asystole is a state of no cardiac electrical activity, hence no contractions of the myocardium and no cardiac output or blood flow...

), or to a flat electroencephalogram, in which the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 shows no electrical activity (brain death

Brain death

Brain death is the irreversible end of all brain activity due to total necrosis of the cerebral neurons following loss of brain oxygenation. It should not be confused with a persistent vegetative state...

). Both of these specific cases are involved in various definitions of death

Death

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

.