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

bits per pixel
bpp
colour depth

    (bpp) The number of bits of information
   stored per pixel of an image or displayed by a graphics
   adapter.  The more bits there are, the more colours can be
   represented, but the more memory is required to store or
   display the image.

   A colour can be described by the intensities of red, green and
   blue (RGB) components.  Allowing 8 bits (1 byte) per
   component (24 bits per pixel) gives 256 levels for each
   component and over 16 million different colours - more than
   the human eye can distinguish.  Microsoft Windows [and
   others?] calls this truecolour.  An image of 1024x768 with
   24 bpp requires over 2 MB of memory.

   "High colour" uses 16 bpp (or 15 bpp), 5 bits for blue, 5 bits
   for red and 6 bits for green.  This reduced colour precision
   gives a slight loss of image quality at a 1/3 saving on
   memory.

   Standard VGA uses a palette of 16 colours (4 bpp), each
   colour in the palette is 24 bit.  Standard SVGA uses a
   palette of 256 colours (8 bpp).

   Some graphics hardware and software support 32-bit colour
   depths, including an 8-bit "alpha channel" for transparency
   effects.

   (1999-08-01)