What does "nano" mean ?

"Nano" comes from the Greek "nanos" meaning "dwarf", is a prefix in science denoting one billionth of a unit.
1 nanometre = one billionth of a metre, i.e. 1:1,000,000,000 metre (10^-9 m).
It is the size of one small molecule, which is 50,000 times smaller than a human hair.
Although a nanometer is very small, there are much smaller units: 1 picometre (10^-12 m), 1 femtometre (10^-15 m), for example, atoms measure approximately 1/10 of a nanometre while their nuclei measure several femtometres.