The
second (symbol:
s) (abbreviated
s or
sec) is the
base unit of
time in the
International System of Units (SI). It is qualitatively defined as the
second division of the
hour by sixty, the first division by sixty being the
minute. It is quantitatively defined in terms of a certain number of periods – about 9 billion – of a certain frequency of radiation from the
caesium atom: a so-called
atomic clock. Seconds may be measured using a mechanical, electric or atomic
clock.