Full Idea
Voluntary euthanasia is done at the request of the person themselves. Involuntary euthanasia is killing someone in their own interests, but disregarding views they could express. Non-voluntary euthanasia is killing someone who cannot express any views.
Gist of Idea
Euthanasia is voluntary (patient's wish), or involuntary (ignore wish), or non-voluntary (no wish possible)
Source
Jonathan Glover (Causing Death and Saving Lives [1977], §15.1)
Book Reference
Glover,Jonathan: 'Causing Death and Saving Lives' [Penguin 1982], p.191
A Reaction
Seems a clear and satisfactory distinction, despite the possibility of borderline cases. A look of pain on a face? An inarticulate person? Deliberate ambiguity? Misunderstanding?