Q. Define amplification factor?

Amplification factor  μ

It is the ratio of the drain -source voltage (ΔVDS) to the change in the gate to source voltage (ΔV_GS) at constant drain current .
Amplification factor,μ ΔV_DS ΔV_GS  at constant ID.

Ampilification factor of a JFET  indicates how much more control  the gate voltage has over drain current than has the drain voltage.
μ = ΔV_DS / ΔV_GS
multiplying the numerator and denominator on R.H.S  by ΔI_D,
we get

μ = (ΔV_DS / ΔV_GS)*(ΔI_D / ΔI_D) = (ΔV_DS / ΔI_D) * (ΔI_D / Δ_GS)

μ = r_d * g_fs

ie

amplification factor = ac drain resistance * transconductance