A unit of time equal to 1/60 of a minute or 1/3600 of an hour.The international definition was originally 1/86400th of a mean solar day in 1900, but is now instead the time required for 9,192,631,770 vibrations of a Cs atom. Since the speed of light is defined to be 299,792,458 m s-1, this makes the meter a derived unit. A mnemonic for the number of seconds in a year is 1 nanocentury is approximately equal to seconds (1 y = s, whereas this approximation gives s).