The Free On-line Dictionary of Computing (30 December 2018):
colour look-up table
(colour look-up table, CLUT) A device
which converts the logical colour numbers stored in each
pixel of video memory into physical colours, normally
represented as RGB triplets, that can be displayed on the
monitor. The palette is simply a block of fast RAM which
is addressed by the logical colour and whose output is split
into the red, green and blue levels which drive the actual
display (e.g. CRT).
The number of entries (logical colours) in the palette is the
total number of colours which can appear on screen
simultaneously. The width of each entry determines the number
of colours which the palette can be set to produce.
A common example would be a palette of 256 colours
(i.e. addressed by eight-bit pixel values) where each colour
can be chosen from a total of 16.7 million colours (i.e. eight
bits output for each of red, green and blue).
Changes to the palette affect the whole screen at once and can
be used to produce special effects which would be much slower
to produce by updating pixels.