Whatever our personal experience and ideologies, perhaps we can agree that the usual understanding of "portrait" includes facial features. Obviously not always two eyes, or we scuttle all the profiles, but noses and mouths are interesting and usually expected.
Though extreme examples of what has been claimed to be portraiture aren't definitive, there is surely room for interpretation, and even semantics.
I don't know if the stellar portrait societies have a definition of "portrait" when they solicit entries for their annual competitions. If it's not in black and white, I'm sure it's in the murky area of judicial preference when the entries are vetted.
My father, and his brother, and their father, had such characteristic posture, walking gaits and hand movements, that I absolutely guarantee that I could paint my dad walking away from me (or even toward me, at a distance from which you couldn't see facial features), and every one of my family members would look at it and say, "That's Dad, isn't it?" As far as I'm concerned, that would be a portrait.
But I'm not trying to convince my family members, or sell to them, and so I'd suggest going with the facial features as the defining variables and constants. If you can agree with a client that a back-of-the-head presentation is the epitome of your subject or loved one, go for it, but see if the deposit check clears before you're too far into it.
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