The simple second has a complex origin: it is measured 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," according to the National Institute of Standards and Technology. This highly technical definition was introduced in 1967 at a meeting of the General Conference on Weights and Measures (CGPM), an international organization.
Prior to 1967, humans used a few different systems of measurement, such as the fraction of 1/86,400 of the mean solar day, but most of these were too unreliable to serve as a standard. In 1960, the CGPM tried breaking down the tropical year into pieces in order to define length of a second, and that stood for a short time. Ultimately, seven years later the atomic reliability of the cesium 133 atom ended up being a much better solution. As of 2015, this is still the official, standard and exact length of a second.