ΔνCs: the hyperfine transition frequency of cesium-133.

• This might sound (and look) like sci-fi jargon, so let’s break this down a
little bit. Cesium is a metal atom in the periodic table. Cesium-133 is its
most common form, or isotope, containing a total of 133 protons and
neutrons.
• Like all atoms, cesium is orbited by electrons. The energy of cesium’s
outermost electron can be controlled with microwave radiation. The
frequency of microwave radiation that causes this electron to jump
between two closely spaced low-energy states is known as the hyperfine
transition frequency.
• The hyperfine transition frequency ΔνCs is equal to 9,192,631,770 hertz.
• In the revised SI, ΔνCs helps to define the second, meter, kilogram and
ampere.