Constant is the scale term in proportionality. Suppose one variable is dependent on the other and one variable become twice, other becomes also twice and if one becomes half, other too becomes half., but they are two different variables which are no-where linked (like cost(money) vs quantity). Quantity and cost are two different things but they abridge a mathematical relation of proportionality. So in the mathematics to abridge these two different variables, we need something. Constant serves that purpose.
