1) One millionth of a second 2) Time unit 3) Unit of time 4) Very short instant 
A microsecond is an SI unit of time equal to one millionth (10−6 or 1/1,000,000) of a second. Its symbol is μs. One microsecond is to one second as one second is to 11.574 days. A microsecond is equal to 1000 nanoseconds or 1/1,000 milliseconds. Because the next SI prefix is 1000 times larger, measurements of 10−5 and 10−4 seconds are typic...
One millionth (10-6) of a second.
(μs) (mi´kro-sek″әnd) one millionth (10−6) of a second.
[n] - one millionth of a second
noun one millionth (10^-6) of a second; one thousandth of a millisecond
One millionth of one second.