The second, symbol , is the SI base unit of time. Its definition fixes the unperturbed frequency of the ground-state atomic transition between levels split by hyperfine structure in caesium with mass number to exactly . One second therefore corresponds to that many cycles of the specified electromagnetic radiation.[ref]
The International System of Units (SI), ninth edition, SI Brochure. §2.3.1: definition of the second through the caesium transition frequency
A practical atomic clock realizes the reference by interrogating atoms and controlling a reference oscillation. Magnetic fields, motion and environmental effects require corrections: an exact unit definition does not make every physical realization free of measurement uncertainty.
Articles by others on the same topic
The term "second" can refer to multiple concepts depending on the context: 1. **Time Measurement**: In terms of time, a second is a unit of measurement equivalent to one-sixtieth of a minute. It is the base unit of time in the International System of Units (SI) and is often used to measure short durations. 2. **Chronological Order**: The term can also refer to the ordinal position in a sequence, indicating the item that comes after the first.
