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.