This short methodological note introduces and summarizes the alpha parameter, a dimensionless measure intended to quantify the non-single-exponential character of photoluminescence decay curves. The note explains the motivation for the parameter, its definition based on two average 'decay' times, and its practical use as a model-independent descriptor of PL decay shapes. It also indicates the original publications in which the conceptual basis of the parameter was proposed and where the alpha parameter was first explicitly defined and applied. The note is intended to increase the visibility and use of this model-independent descriptor among researchers working with luminescence, fluorescence, and photoluminescence decays which have non-exponential character.