The secondary-electron spectrum, or the energy distribution of the ejected electrons resulting from charged-particle impact, is a crucial input to the microscopic modeling of radiation actions. The present article shows a general method for systematizing a bulk of data on the secondary-electron spectrum into a small number of indices. Our method is based on firm theoretical grounds, i.e., on thorough examination of the mathematical physics that governs the secondary-electron spectrum. We present several examples of the analysis of recent experimental data for proton impact.