Austrian Value Theory
The Austrian concept of value at the margin (or marginal utility) is the idea that the value of a good is determined not by its total usefulness or the labor required to produce it, but by the least important use to which an additional (marginal) unit can be put.
Why It Mattered
This overturned the classical economists' labor theory of value. The Austrians showed that value is causally prior to price: people first subjectively value goods at the margin, and those valuations give rise to exchange ratios (prices) in the market.
In short, to the Austrian School, value is an ordinal, subjective ranking of the specific, marginal satisfaction a person expects from one more unit of a good.