What is Marginal Rate Of Substitution?

Legal Definition
In economics, the marginal rate of substitution (MRS) 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.
-- Wikipedia
Legal Definition
Quantification; Measurement: while maintaining the same satisfaction level, how much of one commodity will a consumer give up to get one or more units of another commodity.