Etymology

Today it means: The result of a function on a group of sets, where the function satisfies three conditions: (1) it returns 0 for the empty set; (2) if one set is contained in another set, the function always returns a value that is less than or equal to the result for the containing set; (3) the result for the union of two sets is less than or equal to the sum of the results for each of those sets individually minus the result for their intersection. Full definition →