It is a term used to describe a screen image whose pixel dimensions are the same as the camera dimensions (1:1) On a 19 inch diagonal screen at 1024x768 presentation, it would be like viewing a 40 inch wide print. It is the most critical way to view a digital image. Some people complain that their picture does not look sharp straight out of the camera judged by seeing a 100% crop. Seeing the picture printed at 10x15 inches would be a better method for judging the quality of the image. Your judgment of a 100% crop must be tempered by your understanding of the size of the whole image in relationship to what you see on the screen.