A wavelet is a wave-like oscillation with an amplitude that starts out at
zero, increases, and then decreases back to zero. Like the fast Fourier
transform (FFT), the discrete wavelet transform (DWT) is a fast, linear
operation that operates on a data vector whose length is an integer power
of 2, transforming it into a numerically different vector of the same length.
The wavelet transform is invertible and in fact orthogonal. Both FFT and DWT
can be viewed as a rotation in function space.