Definition of a Second
Until recently, a second was defined in terms of the rotational spin of the Earth, but as the Earth's rotation is slowing down slightly every year, by the late sixties this became problematic for scientists who needed increasingly precise measurements for their experiments.
Today, the second 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."
(Programming Game AI by Example by Mat Buckland, 2005. Page 28-29)
Computers
Computer applications can measure time in seconds or by using the time interval between updates as a kind of virtual second. The latter can simplify many equations, but unless the update rate is locked, the physics will differ between machines of varying speeds. If using a virtual second, make sure your game's physics update frequency is locked to a reasonable rate -- usually the rate of the slowest machine you're developing for.
(Programming Game AI by Example by Mat Buckland, 2005. Page 29)
In the past, the majority of computer games used a fixed frame rate and every component--rendering, physics, AI--was updated at the same frequency. Many of today's sophisticated games, however, specify a unique rate for each component. For example, the physics might be updated 30 times a second, the AI 10 times a second, and the rendering code allowed to go as fast as the machine it runs on.
(Programming Game AI by Example by Mat Buckland, 2005. Page 29)