Definitions for "Marginal Rate of Substitution"
The rate at which one good may be substitutes for another as a consumer moves along their indifference curve i.e. the rate at which consumers are willing to give up units of one good in exchange for more units of another good. Put another way, the MRS of good X for good Y is the amount of good Y that a person is willing to give up to obtain one additional unit of good X.
In a production function or a utility function, the ratio at which one argument (input) substitutes for another along an isoquant or indifference curve.
generally referring to either the rate at which the consumer is willing to substitute one good for another (without loss or gain of satisfaction, see also "indifference curve"); or the rate at which factor inputs can be exchanged in a production process without a change in the production (output) level. (see also "isoquant")