在数学中，特别是测度论中，外测度是一个定义在给定集合上的扩展实数值的函数，并满足几条附加条件。一般的外测度理论由C. Carathéodory引进，目的是给可测集和可数可加测度的理论创建基础。C. Carathéodory关于外测度上所做的工作应用于测度理论中的集合论上（例如外测度用于证明Carathéodory扩张定理）。豪斯多夫也用此来定义一个类似维数的度量，现在称为豪斯多夫维数。

从长度，面积及体积归纳出来的测度概念，对于很多抽象不规则的集合是很有用的。我们希望定义一个广义的测度函数,使其满足以下4个条件：

In mathematics, in particular in measure theory, an outer measure or exterior measure is a function defined on all subsets of a given set with values in the extended real numbers satisfying some additional technical conditions. A general theory of outer measures was first introduced by Constantin Carathéodory to provide a basis for the theory of measurable sets and countably additive measures. Carathéodory's work on outer measures found many applications in measure-theoretic set theory (outer measures are for example used in the proof of the fundamental Carathéodory's extension theorem), and was used in an essential way by Hausdorff to define a dimension-like metric invariant now called Hausdorff dimension.