The more of a good consumed per period, the smaller the increase in total utility from consuming one more unit, other things constant.
the principle that says that as an individual consumes more and more of a good, each successive unit increases her utility, or enjoyment, less and less
Diminishing marginal utility is the decrease in marginal utility that a consumer gets from a product as more of it is consumed. Note that, even if marginal utility is diminishing, so long as the marginal utility is positive, increased consumption leads to increased total utility. It is only when marginal utility is negative that additional consumption reduces total utility.
As we consume more of an item, satisfaction from each extra unit reduces