The Free On-line Dictionary of Computing (30 December 2018):

anti-aliasing

    A technique used on a grey-scale or colour
   bitmap display to make diagonal edges appear smoother by
   setting pixels near the edge to intermediate colours
   according to where the edge crosses them.

   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
   resolution display.  If the display can show intermediate
   greys 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, or an intermediate
   shade of grey according to the proportions of it which overlap
   the black and white areas.  The technique works similarly with
   other foreground and background colours.

   "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.

   (1998-03-13)