constant of variation	Represented by the variable k in variation problems, the constant of variation is a number that relates the input and the output.
direct variation	A type of variation where the output varies directly with the input. Direct variation is represented by the formula y = kx.
inverse variation	A type of variation where the output varies inversely with the input. Inverse variation is represented by the formula .
joint variation	A type of variation where the output varies jointly with multiple inputs. Joint variation is represented by the formula y = kxz.
rational formula	A formula expressed as a rational equation.