A photograph of a person or thing as the main subject.

Selfies are not portraits because it is neither art nor actual photography

Anything can be included in a portrait if it doesn’t change the nature of the work

A good portrait successfully portrays its subject

A portrait cannot just be of a small detail or else it wouldn’t be a portrait anymore

It is no longer a portrait when it becomes more abstract or street photography than a portrait but there isn’t a clear boundary

If it’s a shadow or something that closely resembles the person then it could be, but anything more than that would probably be considered abstraction

It could be a sequence of images if a combination of it makes up a portrait, for example many photos together to make out a single one of a person