The force that is equal in magnitude but opposite in direction to the resultant and achieves equilibrium in a system is called the equilibrant.

Here's a breakdown:

* Resultant force: The sum of all forces acting on an object. It represents the net force.

* Equilibrium: A state where the net force acting on an object is zero, resulting in no acceleration.

* Equilibrant force: The force that, when added to the resultant force, produces a net force of zero, bringing the system into equilibrium.

Think of it like this: imagine you have a bunch of forces pulling on an object in different directions. The resultant force is the "overall" force that's trying to move the object. The equilibrant force is the force that perfectly counteracts the resultant force, keeping the object still.