The Collaborative International Dictionary of English v.0.48:

nano- \nan"o-\ (n[a^]n"[-o]-), pref.
   1. A prefix meaning one-billionth; as, a nanogram is
      one-thousandth of a microgram.
      [PJC]

   2. Very small; submicroscopic; -- used to designate sizes
      smaller than those that would be referred to as micro-;
      as, nanoscale manipulations.
      [PJC]

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

nano-
 pref.

    [SI: the next quantifier below micro-; meaning ? 10^-9] Smaller than 
    micro-, and used in the same rather loose and connotative way. Thus, one
    has nanotechnology (coined by hacker K. Eric Drexler) by analogy with
    microtechnology; and a few machine architectures have a nanocode level
    below microcode. Tom Duff at Bell Labs has also pointed out that ?Pi
    seconds is a nanocentury?. See also quantifiers, pico-, nanoacre, 
    nanobot, nanocomputer, nanofortnight.


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

nano-

   /nan'oh/ 1.  A prefix meaning 10^-9 or one billionth.

   Used loosely to mean "small", e.g. nanotechnology, or
   (rarely), following "nanosecond", to mean a short time,
   e.g. "I'll be with you in a nano".

   [Jargon File]

   (2002-03-02)