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

Animated GIF

    (GIF89a) A variant of the GIF
   image format, often used on web pages to
   provide moving icons and banners.

   The GIF89a format supports multiple "frames" that give the
   impression of motion when displayed in sequence, much like a
   flip book.  The animation may repeat continuously or play
   once.

   Animated GIFs aren't supported by earlier web browsers,
   however the first frame of the image is still shown.

   There are many utilities to create animated GIFs from a
   sequence of individual GIF files.  There are also utilities
   that will produce animated GIFs automatically from a piece of
   text or a single image.

   One problem with this format is the size of the files
   produced, as they are by definition a sequence of individual
   images.  Apart from minimising the number of frames, the best
   way to decrease file size is to assist the LZW compression
   by using blocks of solid colour, avoid dithering, and use
   fewer colours.  If areas of an image don't change from one
   frame to another, they don't need to be redrawn so make the
   area a transparent block in the second frame.

   (1999-08-01)