S plane
In mathematics and engineering, the S plane is the name for the complex plane on which Laplace transforms are graphed. It is a mathematical domain where, instead of viewing processes in the time domain modelled with time-based functions, they are viewed as equations in the frequency domain. It is used as a graphical analysis tool in engineering and physics.
A real function (f) in time 't' is translated into the 's' plane by taking the integral of the function, multiplied by 
from 
to 
, where s is a complex number.
One way to understand what this equation is doing is to remember how Fourier analysis works. In Fourier analysis, harmonic sine and cosine waves are multiplied into the signal, and the resultant integration provides indication of a signal present at that frequency (i.e. the signal's energy at a point in the frequency domain). The 's' transform does the same thing, but more generally. The e-st not only catches frequencies, but also the real e-t effects as well. 's' transforms therefore cater not only for frequency response, but decay effects as well. For instance, a damped sine wave can be modeled correctly using 's' transforms.
's' transforms are commonly known as Laplace transforms. In the 's' plane, multiplying by s has the effect of differentiating in the corresponding real time domain. Dividing by s integrates.
Analysing the complex roots of an 's' plane equation and plotting them on an Argand diagram, can reveal information about the frequency response and stability of a real time system.