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 it is now one of the seven fundamental SI units and is defined as 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 cesium-133 atom (BIPM 1998, p. 95). 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 Eric Weisstein's World of Astronomy is 1 nanocentury is approximately equal to seconds (1 y = s, whereas this approximation gives s).
References
Bureau International des Poids et Mesures. "The International System of Units (SI), 7th ed." 1998. http://www1.bipm.org/en/si/si_brochure/.
