Continuous functions are of utmost importance in mathematics, functions and applications. However, not all functions are continuous. If a function is not continuous at a point in its domain, one says that it has a **discontinuity** there. The set of all points of discontinuity of a function may be a discrete set, a dense set, or even the entire domain of the function. This article describes the **classification of discontinuities** in the simplest case of functions of a single real variable taking real values.

The oscillation of a function at a point quantifies these discontinuities as follows:

- in a
**removable discontinuity**, the distance that the value of the function is off by is the oscillation; - in a
**jump discontinuity**, the size of the jump is the oscillation (assuming that the value*at*the point lies between these limits of the two sides); - in an
**essential discontinuity**, oscillation measures the failure of a limit to exist; the limit is constant.

A special case is if the function diverges to infinity or minus infinity, in which case the oscillation is not defined (in the extended real numbers, this is a removable discontinuity).