/ˈpôrtrət,ˈpôrˌtrāt/ A portrait is a painting, photograph, sculpture, or other artistic representation of a person, in which the face and its expressions are predominant.
I like it better explained as:
An environmental portrait is a portrait executed in the subject's usual environment, such as in their home or workplace, and typically illuminates the subject's life and surroundings. The term is most frequently used of a genre of photography.
By photographing a person in their natural surroundings, it is thought that you will be able to better illuminate their character, and therefore portray the essence of their personality, rather than merely a likeness of their physical features. It is also thought that by photographing a person in their natural surroundings, the subject will be more at ease, and so be more conducive to expressing themselves, as opposed to in a studio, which can be a rather intimidating and artificial experience.