The
second
(
SI
symbol:
s
), sometimes abbreviated
sec.
, is the name of a
unit
of
time
, and is the
International System of Units
(SI)
base unit
of time. It may be measured using a clock.
SI prefixes
are frequently combined with the word
second
to denote subdivisions of the second,
e.g.
, the
millisecond
(one thousandth of a second), the
microsecond
(one millionth of a second), and the
nanosecond
(one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as
kilosecond
(one thousand seconds), such units are rarely used in practice. More commonly encountered, non-SI units of time such as the
minute
(min) and
hour
(h) increase by multiples of 60 and 24 (rather than by powers of ten as in SI).
The second was also the base unit of time in the
centimetre-gram-second
,
metre-kilogram-second
,
metre-tonne-second
, and
foot-pound-second
systems of units.