WordNet (r) 3.0 (2006):

nanosecond
    n 1: one billionth (10^-9) of a second; one thousandth of a
         microsecond

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

nanosecond
ns

    (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)