Q.Define Amplitude Modulation.

Amplitude modulation (AM) is the process in which the amplitude of a high-frequency carrier is varied in accordance with the instantaneous value of the modulating signal, while the carrier's frequency and phase are held constant. The rapid carrier oscillation then sits inside a slowly varying outline called the envelope, whose shape is identical to the original message — which is precisely why the message can later be recovered from the envelope.

To describe it mathematically, take the modulating signal $v_m = V_m \sin(\omega_m t)$ and the carrier $v_C = V_C \sin(\omega_c t)$, with $f_c \gg f_m$. During modulation the carrier amplitude is no longer constant but becomes

so the instantaneous AM voltage is

$v_{AM} = V_C(1 + m_a \sin\omega_m t)\sin(\omega_c t)$ …