Source : Free On-Line Dictionary of Computing

dirty power
     
        Electrical mains voltage that is unfriendly to the delicate
        innards of computers.  Spikes, {drop-outs}, average voltage
        significantly higher or lower than nominal, or just plain
        noise can all cause problems of varying subtlety and severity
        (these are collectively known as {power hit}s).
     
        [{Jargon File}]