**Flux** is a concept in mathematics and physics that can have two meanings. Given a surface in a vector field, flux can refer to
either:

- The net flow of a quantity through the surface or curve (a vector).
- The surface integral or line integral (with respect to the normal rather than tangent vector), which is a scalar.

The second definition is equal to the surface integral of the first definition.

The concept of flux is extremely useful in physics and applied mathematics, especially in electromagnetism and fluid dynamics.