What is descimation?

a Descimator is basically nothing but a down sampler or simply one which reduses the signal sampling rate.the basic working of adecimator can be illustrated with an example eg:consider a signal x(n)={1,2,2,3,1,-3,4,2}and we are told to descimate this by afactor of 2 then then output result would be y(n)={1,2,1,4} which clearely shows that u have reduced the number of samples therby reducing the frequency rate so when we descimate by a factor M we should discard M-1 samples