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.
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.
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