Here is the definition:
What is a persistent vegetative state? According to the National Institute for Neurological Disorders and Stroke people in PVS "have lost their thinking abilities and awareness of their surroundings, but retain non-cognitive function and normal sleep patterns. Even though those in a persistent vegetative state lose their higher brain functions, other key functions such as breathing and circulation remain relatively intact. Spontaneous movements may occur, and the eyes may open in response to external stimuli. They may even occasionally grimace, cry, or laugh. Although individuals in a persistent vegetative state may appear somewhat normal, they do not speak and they are unable to respond to commands." People suffering from PVS can generally be distinguished from afflicted but cognitively intact patients who suffer from "locked-in syndrome" by the fact that "locked in" patients can track visual stimuli and use eye blinks for communication.
According to most neurological experts, Terri Schiavo is definitely PVSher eyes do not really track visual stimuli and she cannot communicate using eye blinks.