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.

In the year 1000 CE, the Persian Muslim scholar al-Biruni first used the term second in Arabic (ثانية thaniyah) and defined it as 186,400 (that is, 1/24 × 60 × 60) of a mean solar day. In the 13th century, scientists who wrote in Latin, including Bacon, and later Kepler and Tycho, used the Latin term parte minutae secundae (or secunda for short) to mean a unit of time which represented the second small part of an hour as the division of one minute by 60 (with the minute being the pars minuta prima or first small part). The use of the word second in English began in the late 16th century. The definition remained unchanged (and still applies in some astronomical and legal contexts) from 1000 until 1960, at which time it was defined as "the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.". However, astronomical observations of the 19th and 20th centuries revealed that the mean solar day is slowly but measurably lengthening, and the length of a tropical year is not entirely predictable either. Thus the sun–earth motion was not considered a suitable basis for the definition. With the advent of atomic clocks, it became feasible to define the second based on a fundamental property of nature. Thus, a mere seven years later in 1967, the International Committee for Weights and Measures (abbreviated CIPM from the French Comité international des poids et mesures) changed the definition to "the duration of 7009919263177000000♠9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." In 1997, the CIPM added that the preceding definition "refers to a caesium atom at rest at a temperature of 0 K."

SI prefixes are 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. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day.

The second is also the base unit of time in other systems of measurement: the centimetre-gram-second, metre-kilogram-second, metre-tonne-second, and foot-pound-second systems of units.

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