American Academy of Neurology Definition of Persistent Vegetative State (PVS)
Incidentially, you can check out a brief synopsis on how PVS is different than brain death as defined by the Harvard Ad Hoc Committee on Brain Death (1968, and chaired by none other than Henry Beecher) and the President’s Commission for the Study of Ethical Problems in Medicine and Biomedical and Behavioral Research, Defining Death. For a more comprehensive (read as a lot longer) read: check out the IOM book Approaching Death: Improving Care at the End of Life (free reading from the Nat'l Ac. Press).
Also, if anyone knows of a web site where I could get a copy of the Harvard's Cmmt's Report, please let me know:-)