The concepts of lifetime and relaxation semiconductors introduced by van Roosbroeck and Casey are reconsidered for amorphous semiconductors and the effect of localized states on the lifetime/relaxation criterion specified: The quantity to be considered is τρ/Td, where Td is (as before) the dielectric relaxation time, but τρ is now the lifetime of the total charge including charge stored in localized states. τρ is equal to the free carrier lifetime times a correction factor that is much larger than unity for amorphous semiconductors. Bandtail states and dangling bonds act differently on the criterion. The correction factor is frequency-dependent and decreases at higher frequencies. Two illustrative experimental examples (low substrate temperature a-Si:H, a-SiC:H alloys) are given, showing borderline cases.