There's a cave near Angoulême in western France whose wall is decorated with the oldest known portrait. It was painted at least 27,000 years ago. Obviously, we have no idea who created it and no idea who the subject was — but it's recognizable as a portrait.
Most often a portrait depicts the recognizable likeness of somebody (or some creature) who is important to us, but it could be anybody. Spouse, stranger, public figure, animal companion. A good portrait conveys something of the mood or personality of that being. That's one advantage of using a mobile device for portraiture; nobody is threatened by a cellphone camera (well, except for the police).