In answer to the original question (one answer, anyway), as "paint on a canvas" is a painting, a "tree in a pot" is a bonsai.
As to the point at which a bonsai is "legitimate," that is going to vary according to each individual's standards. Those of long-time training and depth of understanding are going to have different standards from those with little training and understanding. This is only natural.
I agree with Matt, it's a moving target. Little is accomplished or defined by trying to nail down such a thing.
Kind regards
Andy Rutledge
www.bonsai365.com/
zone 8, Texas