a combining form with the meaning “very small, minute,” used in the formation of compound words (nanoplankton ); in the names of units of measure it has the specific sense “one billionth” (10 -9 ): nanomole; nanosecond.
/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)