second, n.All

Clocks were, before 1687, made with only an hour hand.

A second is called a second because it's the result of the second division of an hour by 60.

One second is technically the duration of 9,192,631,770 oscillations of a Caesium 133 atom's outermost electron.

A second is quick, but a nanosecond is one billionth of a second

The Second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.