I recently participated in a "faceless portrait" challenge. Some of the photos looked like decapitations. Others were people with there faces covered with a cloth mask, or a hand in front of the face. If there is such a thing as faceless portraits, what are the criteria?
I'd like to think that Yousuf Karsh's 1954 picture of cellist Pablo Casals counts as a portrait. When arguably the best portraitist of his age believes that such a representation of his subject is the best way of conveying that person's essential identity to others, I tend to give that opinion some weight. Much the same could be said about a picture of a shy child hiding behind her hands (or, in a former time, hiding in her mother's skirt), an artist at the easel, dad (or grandpa) being little more than a pair of hands supporting a newspaper at the table, or a student lost in a book.
That doesn't make any old picture of a person whose face you can or cannot see a "portrait". A portrait ought to extend beyond a mere likeness into the realm of essence. The Sears and J.C. Penney catalogs, fashion and beauty magazines, and so on, are full of likenesses that have nothing to do with portraiture. On the other hand, the Farm Security Administration project and Life and National Geographic magazines have generated an awful lot of genuine portraits, even if the individuals in the pictures can't be (and aren't meant to be) identified.
It's that feeling you get that you know something about the person that makes the difference. (Whether what you know about the person is something genuine or merely a persona they're projecting for the picture is another question entirely. Are they pictures of Hal Holbrook and James Whitmore in costume, or portraits of portaits of Mark Twain and Harry Truman? Things can get a bit fuzzy around the edges.)