I recently paid good amount of money to have portraits of my girlfriend and I made. For this, we chose a local photographer who is skilled and who could take photographs that represent US. He is actually a nationally recognized person in portraiture.
What we didn't want was, "Look here, smile, snap. Look there now, smile, snap. Put your arm around her, snap" type of pictures. Our criteria was high as I fancy myself as an amateur portrait photographer. (go ahead and laugh, if you want...)
This is what portraits (of us) are for me: Photographs that represents US. Not just an accurate depiction of how we look but how we feel about us individually and as a couple together placed on paper. Yes, the images were technically superior to anything I could ever done. Poses, lighting, etc, etc, etc, were just perfect. Yes, the images were subtly altered. But only to an extent to reflect how we feel about ourselves. But best of all, they captured who we are.
I see many pictures of people that doesn't tell what kind of people they are, how they feel, and basically emotionless. TO ME, those are not portraits.
I just came in to this thread to give MY definition of portraiture. Everybody is free to agree or disagree.
Oh, another thing I believe in photography is, if I have to explain it, it failed. I think this applies to portraiture as well in more ways than one.