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.
2. Very small; submicroscopic; -- used to designate sizes
smaller than those that would be referred to as micro-;
as, nanoscale manipulations.
The Jargon File (version 4.4.7, 29 Dec 2003):
[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):
/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".