What is modulation index in AM and how does it affect transmission?

AM modulation index m = Em/Ec (message amplitude / carrier amplitude), also expressed as percentage. For m < 1: under-modulation, envelope follows message without distortion. For m = 1: 100% modulation, maximum efficiency without distortion. For m > 1: over-modulation causes envelope clipping and harmonic distortion. Power efficiency = m^2/(2+m^2), reaching 33% at m=1 for single-tone modulation. Sideband power is Psb = (m^2/4)*Pc. Proper modulation index balances power efficiency with distortion avoidance.