Nano- (symbol n) is an SI prefix meaning one billionth. Used primarily with the metric system, this prefix denotes a factor of 10−9 or 0.000000001. It is frequently encountered in science and electronics for prefixing units of time and length.
One nanometer is about the length that a fingernail grows in one second.
Three gold atoms lined up are about one nanometer long.
If a toy marble was one nanometer wide, Earth would be about one meter (3.3 feet) wide. 
The prefix is derived from the Greek νᾶνος, meaning "dwarf", and was officially confirmed as standard in 1960.