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The initial fluctuation spectrum is a power spectrum (angular power spectrum), representing the density fluctuations in the early universe (initial fluctuations aka primordial fluctuations) hypothesized to be the origin of the variation in the cosmic microwave background (CMB) (CMB anisotropies) as well as the uneven density of the present universe. Working backward, there must have been some variation in density in the early universe to result in today's large scale structure, galaxy clusters, galaxies, etc., and the CMB anisotropies are presumed to be our view of those variations at the time of recombination. Cosmic inflation is an element of the current working model explaining the origin of this initial uneven density.