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In mathematics, a set function is a function whose input is a set. The output is usually a number. Often the input is a set of real numbers, a set of points in Euclidean space, or a set of points in some measure space.
Examples

Examples of set functions include:

The function that assigns to each set its cardinality, i.e. the number of members of the set, is a set function.
The function

$$d(A) = \lim_{n\to\infty} \frac{|A \cap \{1,\dots,n\}|}{n},$$

assigning densities to sufficiently well-behaved subsets A ⊆ {1, 2, 3, ...}, is a set function.

The Lebesgue measure is a set function that assigns a non-negative real number to each set of real numbers. (Kolmogorov and Fomin 1975)
A probability measure assigns a probability to each set in a σ-algebra. Specifically, the probability of the empty set is zero and the probability of the sample space is 1, with other sets given probabilities between 0 and 1.
A possibility measure assigns a number between zero and one to each set in the powerset of some given set. See possibility theory.
A Random set is a set-valued random variable. See Random compact set.

References

A.N. Kolmogorov and S.V. Fomin (1975), Introductory Real Analysis, Dover. ISBN 0-486-61226-0

Sobolev, V.I. (2001), "Set function", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
*Regular set function at Encyclopedia of Mathematics