define secondly
The second is also the base unit of time in other systems of measurement: the centimetre-gram-second, metre-kilogram-second, metre-tonne-second, and foot-pound-second systems of units.n 1956, the second was redefined in terms of a year (the period of the Earth 's revolution around the Sun) for a particular epoch because, by then, it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time.