Definition of Measure: Measure is a noun, in the mathematical language of measure theory: a measure is a function from sets to the real line. Probability is a common kind of measure in economic models. Other measures are the counting measure, which is the number of elements in the set, the length measure, the area measure, and the volume measure. Length, area, and volume are defined along lines, planes, and spaces just as one would expect, and they have the natural meanings.

Formally: a measure is a mapping m from a sigma algebra A to the extended real line such that

1. m(null) = 0
   
2. m(B) >= 0 for all B in A
   
3. m(any countable union of disjoint sets in A) = the sum of m(each of those sets)