The Jargon File (version 4.4.7, 29 Dec 2003):

line noise
 n.

    1. [techspeak] Spurious characters due to electrical noise in a
    communications link, especially an RS-232 serial connection. Line noise may
    be induced by poor connections, interference or crosstalk from other
    circuits, electrical storms, cosmic rays, or (notionally) birds crapping
    on the phone wires.

    2. Any chunk of data in a file or elsewhere that looks like the results of
    line noise in sense 1.

    3. Text that is theoretically a readable text or program source but employs
    syntax so bizarre that it looks like line noise in senses 1 or 2. Yes,
    there are languages this ugly. The canonical example is TECO; it is often
    claimed that ?TECO's input syntax is indistinguishable from line noise.?
    Other non-WYSIWYG editors, such as Multics qed and Unix ed, in the hands
    of a real hacker, also qualify easily, as do deliberately obfuscated
    languages such as INTERCAL.


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

line noise

    1. Spurious characters due to electrical
   noise in a communications link, especially an EIA-232
   serial connection.  Line noise may be induced by poor
   connections, interference or crosstalk from other circuits,
   electrical storms, cosmic rays, or (notionally) birds
   crapping on the phone wires.

   2. Any chunk of data in a file or elsewhere that looks like
   the results of electrical line noise.

   3. Text that is theoretically a readable text or program
   source but employs syntax so bizarre that it looks like line
   noise.  Yes, there are languages this ugly.  The canonical
   example is TECO, whose input syntax is often said to be
   indistinguishable from line noise.  Other non-WYSIWYG
   editors, such as Multics "qed" and Unix "ed", in the
   hands of a real hacker, also qualify easily, as do
   deliberately obfuscated languages such as INTERCAL.

   [Jargon File]

   (1994-12-22)