1) One quadrillionth of a second 2) Time unit 3) Unit of time 
A femtosecond is the SI unit of time equal to 10−15 of a second. That is one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31.7 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a v...
One quadrillionth (10-15) of a second of a second
[n] - one quadrillionth of a second
femtosecond A femtosecond is one millionth of a nanosecond or 10-15 of a second and is a measurement sometimes used in laser technology.
Abbreviated as fs, unit of time equal to 10-15s. See also: Decimal Prefixes, Second.
noun one quadrillionth (10^-15) of a second; one thousandth of a picosecond