Femtosecond

A femtosecond is 10^-15 seconds. A femtosecond compared with a second is like the thickness of the letter "I" on a sheet of paper compared to the Earth-Sun distance (150 million km). If light can travel around the earth eight times in one second, it only travels 30 microns (the thickness of a human hair) in 100 fs.

Sub-multiple: the attosecond.