(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 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). Admiral Grace Hopper famously handed out foot-long lengths of wire to students to illustrate how far an electrical signal can travel in a nanosecond.
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 microsecond (us or Greek letter mu plus s) is one millionth (10-6) of a second.
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.
See also: Grace Hopper nanosecond.