We propose (P cyc /P rot ) 2 , the square of the ratio of the characteristic oscillatory timescales of stellar chromospberic activity to the rotation period, as a useful parameterization of the stellar activity cycle and as the observational equivalent of the theoretical dynamo number, N D . (P cyc /P rot ) 2 can be obtained observationally from 25 yr activity records of stellar Ca II H and K chromospheric emission fluxes of the Mount Wilson Observatory HK Project