What is a Spectrum

This contribution describes the "spectro-perfectionism" algorithm of Bolton & Schlegel (2010, PASP, 122, 248) that is being implemented within the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III (SDSS-III), in terms of its potential to deliver Poisson-limited sky subtraction and lossless compression of the input spectrum likelihood functional given raw CCD data.