在经典力学里，牛顿第三定律（Newton's third law）阐明，当两个物体相互作用时，彼此施加于对方的力，其大小相等、方向相反。力必会成双结对地出现：其中一道力称为「作用力」；而另一道力则称为「反作用力」（拉丁语 actio 与 reactio 的翻译），又称「抗力」；两道力的大小相等、方向相反。它们之间的分辨，是纯然任意的；任何一道力都可以被认为是作用力，而其对应的力自然地成为伴随的反作用力。这成对的作用力与反作用力称为「配对力」。牛顿第三定律又称为「作用与反作用定律」，在本文内简称为「第三定律」。

As described by the third of Newton's laws of motion of classical mechanics, all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. The third law is also more generally stated as: "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts." The attribution of which of the two forces is the action and which is the reaction is arbitrary. Either of the two can be considered the action, while the other is its associated reaction.