Anti-aliasing

Anti-aliasing is a display technique used on grayscale or color raster graphics to make diagonal edges appear smoother by setting pixels near an edge to intermediate values according to where the edge crosses them.

Description
The most common example is black characters on a white background. Without anti-aliasing, diagonal edges appear jagged, like staircases, which may be noticeable on a low display. If the display can show intermediate grays then anti-aliasing can be applied. A pixel will be black if it is completely within the black area, or white if it is completely outside the black area; an intermediate shade of gray can be determined by calculating the proportions of it contained within the black and white areas. The technique works similarly with other foreground and background colors.

Aliasing refers to the fact that many points (which would differ in the real image) are mapped or "aliased" to the same pixel (with a single value) in the digital representation.