Originally Posted by Bob Michaels
* A portrait is an image whose specific purpose is to convey information about an individual or individuals. Not their environment, not their clothing, not some specific object in the image, but them.
* There is some level of agreement and cooperation between the subject and the artist. This can be momentary but the subject must be aware.
With the first suggestion I agree fully, but with the second absolutely not. I am sure that you can make very good portraits of people that are not aware of being portrayed, not only on photographs, but also sculpted or painted. Many examples prove this.