The term to describe diagonal lines in a digital image that appear jagged. Jaggies are produced due to the square nature of a pixel. Even when aliased certain subjects like telephone lines seem unnatural. It can be made worse by sharpening images.
Look at the picture of a Motorhome above. Virtually all the straight edges are jagged. However, this was a screen shot of a monitor. The original image was actually fine when zoomed in. The lesson there is to not evaluate an image without a close look. You might delete perfectly good images.
Related Terms...
Aliasing - An effect caused by sampling an image (or signal) at too low a rate. It makes rapid change (high texture) areas of an image appear as a slow change in the sample image. Once aliasing occurs, there is no way to accurately reproduce the original image from the sampled image.
Anti-aliasing - The process of reducing stair-stepping by smoothing edges where individual pixels are visible.