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.