A wavelet is a wave-like oscillation with an amplitude that starts out at zero, increases, and then decreases back to zero.
A wavelet can typically be visualized as a "brief oscillation" like one might see recorded by a seismograph or heart monitor. Generally, wavelets are purposefully crafted to have specific properties that make them useful for signal processing. Wavelets can be combined, using a "revert, shift, multiply and sum" technique called convolution, with portions of an unknown signal to extract information from the unknown signal.
Source: Wikipedia.
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