Source : WordNet®

nanosecond
     n : one billionth of a second; one thousandth of a microsecond

Source : Free On-Line Dictionary of Computing

nanosecond
     
         (ns) 10^-9 seconds; one thousand millionth part of a
        second.
     
        This is the unit in which the fundamental logical operations
        of modern digital circuits are typically measured.  For
        example, a {microprocessor} with a {clock} frequency of 100
        {megahertz} will have a 10 nanosecond clock period.
     
        (1996-11-15)