SIsekunden

SISekunden is not an official term in the International System of Units (SI). In German-language contexts, the expression may appear informally to denote a "second according to the SI definition" or to distinguish SI seconds from non-SI time units. The more precise and widely used term is simply the second, with the symbol s. The potential use of SISekunde reflects the emphasis on SI-traceable time measurements in certain technical discussions, rather than a separate unit.

Definition and basis. The SI second is the base unit of time in the International System of Units. Since 1967 (with refinements over time), its modern definition ties to the cesium-133 atom: one second is defined as exactly 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the atom’s ground state. This definition provides a highly reproducible and universal standard independent of celestial motions.

Realization and usage. Time standards are realized through atomic clocks and distributed time scales, such as International Atomic Time (TAI) and Coordinated Universal Time (UTC). UTC is kept close to UT1, which reflects Earth’s rotation, by occasional leap seconds. SI seconds underpin precise time stamping, synchronization of communication networks, navigation systems, science experiments, and many industrial processes. All SI-based timing relies on the precise realization of seconds rather than any alternative second concept.

Notes. Although SISekunde may appear in some texts, it remains an informal or contextual expression. The official concept and symbol to use is the second (s) as defined by the SI. See also SI base units, atomic clocks, TAI, UTC, and time measurement.