One of the pair of forces described in Newton's third law.
A mutual action between objects where each object exerts an equal and opposite force on the other.
Newton's third law
Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first.
The force that is equal in strength and opposite in direction to the action force, and one that acts simultaneoisly on whatever is exerting the action force.