边际替代率（Marginal rate of substitution）是指在甲乙两种商品组合及保持效用不变的条件下，消费者每得到额外一单位商品甲之后要而必须放弃的商品乙的数量。
在帕累托最优情况下，任意两种商品的边际替代率是相同的。
英语百科
Marginal rate of substitution 边际替代率
In economics, the marginal rate of substitution is the rate at which a consumer is ready to give up one good in exchange for another good while maintaining the same level of utility. At equilibrium consumption levels (assuming no externalities), marginal rates of substitution are identical.