What is “nano”?

Before writing about nanotechnology and nanomaterials I need to explain:
What is “nano”?

My friend Chris always says: do not think that iPod nano is a really nano sized device :)

Then, how big is one nanometer?

1 nanometer (nm) is 1 billionth of a meter (m)

In other words:
One nanometer is to a tennis ball what a tennis ball is to the Earth