Definition - What does Femtosecond mean?
A femtosecond is one quadrillionth of a second. It also can be expressed as 1,000 picoseconds or 1,000,000 nanoseconds.
Techopedia explains Femtosecond
This extremely short period of time is part of an emerging system of observations for transfers related to most current technology. In one respect, the femtosecond is not practically useful in a number of situations in which microprocessor speed or data transfers are measured.
To get an idea of a femtosecond's actual scale, keep in mind that with the speed of light at 300,000km per second, light travels about 30cm in one nanosecond. A femtosecond is 1,000,000 nanoseconds, which means that in one femtosecond, light travels 300 nanometers - a distance in the microscopic range.