The basic problem of harmonic analysis is the decomposition of elements in some topological vector space of functions in some linear basis which is typically distinguished by some nice representation theoretical properties. This decomposition can be a sum, and a basis a topological basis, but more general it is a decomposition in the sense of an integral. The elements of the distinguished bases were in historical examples thought of as “basic waves” or “harmonics”. Some standard examples are Fourier analysis on locally compact abelian groups, wavelet analysis?, quantum group Fourier transform etc. In some cases the elements of the “basis” are not linearly independent, e.g. in the case of decomposition into coherent states.
Peter-Weyl theorem?,