As I sat in my high school Anatomy & Physiology class, I distinctly remember our teacher, Mrs. Nesselroth telling us that it was the absence of brain waves that is the definitive threshold to death. That is the point of no return.
We bombarded her with questions of ‘heart stopping’ and the like, but there have been instances of coming back from those. The one definitive line is the absence of brain waves. It’s now literally decades later and I still remember that lesson.
Did I mention that in high school I entertained the idea of becoming a doctor?