In many books I've seen the generalized transfer function expressed using:
\$\displaystyle G(s) = K \frac{(s + z_1)(s + z_2)\ldots}{s^n (s + p_1) (s + p_2)\ldots} \$
What I don't understand is where the lonely \$s^n\$ term in the denominator comes from? I've searched high and low but I've not found any explanation for this term.