- Marginal Rate of Substitution (MRS): Rate at which a consumer gives up one good for another to maintain constant utility. - Marginal Rate of Technical Substitution (MRTS): Rate at which a firm substitutes one input for another to maintain constant output. - Diminishing Marginal Utility: The concept that the additional utility gained from consuming one more unit of a good decreases as consumption increases. - Equi-marginal utility: Condition for consumer equilibrium where the ratio of marginal utility to price is equal for all goods.