Marginal utility
Etymology
“Marginal” derives from Latin “margo” (“edge, border”), here the edge of one more increment; “utility” from Latin “utilis” (“useful”). The concept anchored the “marginal revolution” of the 1870s, when Jevons, Menger, and Walras independently reframed value around the last unit consumed rather than total worth.
noun phrase · /MAR-jin-ul yoo-TIL-i-tee/
The added satisfaction gained from one more unit of something, which typically shrinks as you get more.
What it names
Why the value of the next unit — not the total — drives decisions, and why the first slice matters more than the fifth.
Example
The tenth cup of coffee gives you far less marginal utility than the first — and may even make you jittery.
Related idea
Marginal utility is the benefit of the last unit; diminishing returns is the broader pattern of each added input yielding less output, whether or not utility is involved.