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,” or one thousand seconds), such units are rarely used in practice. More commonly encountered, non-SI units of time such as the minute, hour, and day 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.

International second

Under the International System of Units, the second is currently defined as quote: The second is the duration of nowrap: 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.Official BIPM definition

This definition refers to a cesium atom at rest at a temperature of 0 K (absolute zero), and with appropriate corrections for gravitational time dilation. The ground state is defined at zero magnetic field. The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. (See History]] below.)

The international standard symbol for a second is s (see [[ISO 31-1)

The realization of the standard second is described briefly in NIST Special Publication 330; Appendix 2, pp. 53 ff, and in detail by National Research Council of Canada.

In practice, the transition is measured in the quantum vacuum where vacuum fluctuations can lead to shifts in atomic energy levels in vacuum relative to their values in free space, for example, to a Lamb shift. Consequently, the transition in quantum vacuum may not have the same frequency as in free space. Free space (which, like absolute zero, is a theoretical reference state that cannot be attained in practice, with exact values for its electromagnetic properties: c₀, μ₀, ε₀, and Z₀) is the reference state for the SI units for the metre and ampere.