Answer:
Portrait orientation is about whether the portrait subject will be pictured the long way, like in the vertical position, or whether the subject will be shown with the canvas or paper in a horizontal position (the subject of the portrait won't necessarily by lying down, but just that the portrait surface would be in the horizontal position)
Another Answer
Commonly, in computer printing, portrait orientation infers that the left side of the printed page is 11-1/2 inches long. It's opposite is landscape orientation, which infers that the left side of the page is 8-1/2 inches long.