| the extra utility or satisfaction derived by a consumer from the consumption of the last unit of a commodity. |
Marginal Utility
The additional satisfaction a consumer gains from consuming one more unit of a good or service.
Investopedia Commentary
For example, if you were really thirsty you'd get a certain amount of satisfaction from a glass of water. This satisfaction would probably decrease with the second glass, and then even more with the third glass. The additional amount of satisfaction that comes with each additional glass of water is marginal utility.
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See also: Economics, Law of Diminishing Marginal Utility, Total Utility, Utility