What is the extra satisfaction from consuming one additional unit of a good called?

The extra satisfaction from consuming one additional unit of a good is called marginal utility.

Marginal utility is the change in total utility resulting from one more unit of consumption. In the traditional cardinal approach, utility represents satisfaction and may be expressed in hypothetical units. If consuming another unit adds less satisfaction than the previous unit, marginal utility is diminishing.

The law of diminishing marginal utility is an important explanation for a downward-sloping demand curve. As a person consumes more units of a good during a period, additional units may be less valuable, so the person may buy extra units only at lower prices. This is a general tendency, not a universal rule for every experience or product.

Marginal utility is different from total utility, which is the overall satisfaction from all units consumed. It is also different from marginal cost, which concerns the additional cost of producing one more unit. Modern economics often uses ordinal utility, ranking preferences rather than assigning precise numerical satisfaction values.