When a line or any shape (curve, circle or font text character) is painted, and its edges are not perfectly horizontal or vertical, some pixels are only partially covered. The resulting jagged-edged lines are said to be aliased. Aliasing gives lines a "stair-step" or "jaggie" appearance. The greater the number of dots or pixels on your display, the higher the display resolution, and the less noticeable aliasing is to the human eye.

This definition is in context to Photography. See more contextual defintions for Aliasing.