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

phase alternating line

    (PAL) The video signal format used in the UK
   [where else?].

   PAL uses Amplitude Modulation for the video information, and
   Frequency Modulation for the audio information.  The phase
   of the colour subcarrier is reversed on alternate lines
   which (together with the use of a delay line) allows the
   receiver to cancel any phase errors introduced in the path
   between the studio and the end-user's receiver.  Such phase
   errors are quite common and would cause the displayed colours
   to shift in hue.

   The US equivalent, NTSC, does not have this feature and thus
   requires a user control to correct for transmission phase
   errors, hence the mis-expansion "Never Twice the Same Colour".

   (2001-06-08)