Newton didn't specifically give a name to the force exerted by the second object back on the first. This is because he described it using his Third Law of Motion, which states:

"For every action, there is an equal and opposite reaction."

This means that the force exerted by the first object on the second (the "action") is equal in magnitude and opposite in direction to the force exerted by the second object on the first (the "reaction").

Therefore, it's not about a distinct name but about understanding that these forces are always paired and equal in magnitude and opposite in direction.