It's when nothing (more) can be done to help the patient.
It's something I have to translate (French into English), and it doesn't apply to a human being, but to the earth, many years from now, when it's about to ... burst,...stop existing? ...disintegrate?
In French, it's called "Phase terminale" (terminal step, phasis, condition)
I don't think "incurable" will do the trick...
Thanks, Antonia!