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The Metric System is a common term for the International System of Units.

In mathematics, a metric (also called distance metric) is an abstraction of the notion of distance; it is used to define metric spaces. The article on metric spaces contains a detailed discussion of the topic.

An important special case is the metric that turns a manifold into a Riemannian manifold. Such a metric allows to introduce geodesics and to measure distances and angles on the manifold.