A marginal value is

1. a value that holds true given particular constraints,
2. the change in a value associated with a specific change in some independent variable, whether it be of that variable or of a dependent variable, or
3. [when underlying values are quantified] the ratio of the change of a dependent variable to that of the independent variable.

(This third case is actually a special case of the second).

In the case of differentiability, at the limit, a marginal change is a mathematical differential, or the corresponding mathematical derivative.

These uses of the term “marginal” are especially common in economics, and result from conceptualizing constraints as borders or as margins. The sorts of marginal values most common to economic analysis are those associated with unit changes of resources and, in mainstream economics, those associated with infinitesimal changes. Marginal values associated with units are considered because many decisions are made by unit, and marginalism explains unit price in terms of such marginal values. Mainstream economics uses infinitesimal values in much of its analysis for reasons of mathematical tractability.