Flux measures how much of something (a field, fluid, or force) passes through a given surface at a specific moment in time.

Think of it like counting how many bananas fly through a window. The total depends on how strong the banana-throwing field is, how big your window is, and whether the window faces the bananas head-on or at an angle.

In physics and vector calculus, you'll see flux pop up in Gauss's Law (electric fields), Faraday's Law (magnetic fields), and fluid dynamics. It's calculated by multiplying the field strength by the surface area and the cosine of the angle between them.

The formula looks scary (surface integrals, dot products), but the intuition is simple: perpendicular surfaces catch maximum flux, parallel surfaces catch zero.