The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).[1] [2] It is qualitatively defined as the second division of the hour by sixty, the first division by sixty being the minute.[3] 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.