The force that opposes or restricts motion in a fluid is called viscosity.

Here's why:

* Viscosity is a measure of a fluid's resistance to flow. Think of it as the "stickiness" or "internal friction" of the fluid.

* High viscosity means the fluid flows slowly and resists movement (like honey or syrup).

* Low viscosity means the fluid flows easily and readily (like water or air).

Let me know if you want to know more about viscosity!