A dynamic reactivity, not the reactivity in general use, is defined relative to prompt critical as ΔK = −lα, where α is the asymptotic (prompt) flux decay rate observed in a pulsed neutron experiment, and l is the prompt generation time of that same reactor made prompt critical by uniform subtraction of 1/υ poison. The dynamic reactivity coalesces near critical with the conventional perturbation reactivity δν/ν. The dynamic reactivity is physically interpretable as the amount of uniform 1/υ poison whose removal would result in criticality, times the conventional reactivity coefficient of that poison in the critical reactor. The quantity l has the physical significance of the average time taken by a neutron to cause a fission in the steady-state prompt-critical reactor; l is also the reactivity coefficient just mentioned.