The second (symbol: s), sometimes abbreviated sec., is the name of a unit of time. It is one of the seven SI base units. That means that the second is used to build the foundations of many other units (e.g. speed is a meter per second, in SI units). One second is officially measured as the time taken by 9,192,631,770 cycles of radiation that comes from electrons moving between two energy levels of the caesium-133 atom.