In chemistry "Kc" represents the equilibrium gas constant in terms of molar concentrations. It is related to the other equilibrium gas constant, Kp, which is used for partial pressures. In both constants, the second letter is written in subscript.
When a gas reaction is in equilibrium, the reaction undergoes no net changes, because the reaction is occurring in both directions at the same time. The value of Kc is found by multiplying the concentration of the products taken to the power of their coefficients and dividing that by the product of the reactants, similarly taken to the power of their coefficients. For example, in the reaction H2 + I2 ? 2HI, the formula for Kc is [HI]^2/([H2][I2]).