What name is given to a force pair described by Newton's third law?

A force pair described by Newton’s third law is called an action-reaction pair.

Newton’s third law says that when one body exerts a force on another, the second body exerts an equal force in the opposite direction on the first. The two forces arise together and act on different objects, which is why they do not simply cancel each other on one free-body diagram.

For example, Earth pulls a falling apple downward while the apple pulls Earth upward. The forces have equal magnitude, but Earth’s vastly greater mass gives it a much smaller acceleration. The same principle explains walking, swimming and rocket propulsion: a person pushes the ground, a swimmer pushes water, or a rocket pushes exhaust backward.

A common mistake is to call the pair an equilibrium pair. Equilibrium concerns forces acting on the same object whose vector sum is zero; a third-law pair acts on two different objects. “Action” and “reaction” are also labels rather than a strict time sequence: either force may be named first.