(OOC sorry for copying this straight from my post on the WAGA thread on this)
Mayhaps, at least, if you want a definition for Terminally Ill, a suggestion would be: "A person who is suffering from a life-threatening illness to which there is no present cure, or present cures do not work on the person." Might need to be slightly reworded, but it could cover those which treatment has failed to work on (I.E: their own immune system prevents it, or the nature of their species.