In digital signal processing, decimation is a technique for reducing the number of samples in a discrete-time signal. The element which implements this technique is referred to as a decimator.
Decimation is a two-step process:
An example of decimation: the frequency of a recorded sound can be raised an octave (in other words, doubled in frequency) by eliminating every other sample without changing the sampling rate. This will result in aliasing if the sound contains overtones whose (doubled) frequency will exceed half the sampling rate. Decimation aliasing can be avoided by eliminating those overtones with a lowpass filter before downsampling.
The same principle applies to eliminating samples at other intervals.