A photograph is a portrait if it features a person as the primary subject. Formal portraits are usually done with from the waist or shoulder up, and they are photographs that are carefully planned. Facial expressions and body language are the primary focuses of a portrait, although other aspects like clothing and location can also be of significance. In my opinion, there is anything that should or shouldn’t be included in a portrait, because each can contribute to the meaning of the photograph.
I think that selfies are portraits because they accomplish the basic idea of taking a photo of a subject. However they cannot be considered formal portraits because the angle will often be strange and different compared to somebody else taking the photo. Furthermore, selfies are usually taken on phones or other technologies that have a poorer quality front-facing camera. Photos taken by oneself are all portraits as long as the main subject is of a person, but a portrait taken with a phone by holding up a selfie-stick will be drastically different from a portrait using a professional camera, tripod, and the timer feature.
A good portrait should tell a story. There are many choices in the making of a portrait, and each decision can contribute to a certain kind of message. For example, choosing to have the eyes looking into the camera versus into the distance can change the meaning, with one feeling defiance while the other dreaming. Considering certain angles, props, or framing can significantly enhance the message and make it more apparent. They can help the audience feel stronger emotions.
I think that a portrait could consist of a small detail only, especially if it is displayed as a sequence of images. The hands of a chef might look different from one of a violinist, and a close-up photograph allows for these differences to stand out. I think that an abstracted representation only ceases to become a portrait if human details can no longer be told apart from everything else. Therefore, an arrangement of inanimate objects would not be considered a portrait.