Photorealism is the manual creation of an image to look like a photograph from a camera. For example, if a hand-drawn or painted image looks like it was taken by a camera, the image is photorealistic.
Photorealism in CGI
In the field of CGI (computer-generated imagery), computers simulate the behavior of light, and its interaction with virtual objects, in a process called ray tracing. CGI can produce high quality photorealistic images, but takes a great deal of time and processing power.