Photorealism is a style of drawn or painted art that goes so far beyond the realist school that it is hard to distinguish it, visually, from a photograph. Such a definition is from the perspective of a non-photographer, such as Louis K. Meisel, who often is credited with originating the term.
It also needs to be put into the context of a photographer who uses photographic techniques to make the image more attractive, or at least different, than what the eye would see. A photorealist painter, for example, might paint a landscape, but use her brushes and pigments to make a nondescript sky more blue, with more sharply delineated clouds. A color photographer, however, also can make that sky more dramatic by using a polarizing filter to reduce haze.
Pin-up art might show a model with apparently flawless skin, but blemishes could be absent because a photorealist painter chose not to include them, because a makeup artist covered them before a photograph was taken, or the photograph was manually or digitally retouched to remove them. Skilled visual examination may be able to distinguish among these, or a more technical approach might be employed. 
Many non-photographic artists start with a photograph, such as Ralph Goings who said, "What I'm about is making paintings, and my camera is one of the tools I use. It's the artist's job to take the painting beyond the photograph."
- Florin Cutzu, Riad Hammoud, Alex Leykin (2003), "Estimating the photorealism of images: Distinguishing paintings from photographs", 2003 IEEE Computer Society Conference on Computer Vision and Pattern Recognition (CVPR '03) - Volume 2
- Claire O'Neill (15 October 2009), Really Real Photorealism, NPR