(This definition follows U.S. usage in which a billion is a thousand million and a trillion is a 1 followed by 12 zeros.)
A microsecond (us or Greek letter mu plus s) is one millionth (10 -6 ) of a second.
For comparison, a millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet.
A nanosecond (ns or nsec) is one billionth (10 -9 ) of a second and is a common measurement of read or write access time to random access memory ( RAM ).
A picosecond is one trillionth (10 -12 ) of a second, or one millionth of a microsecond.
A femtosecond is one millionth of a nanosecond or 10 -15 of a second and is a measurement sometimes used in laser technology.
An attosecond is one quintillionth (10 -18 ) of a second and is a term used in photon research.