A Lagrange point is a position in space where the gravitational forces of two large bodies, such as a planet and a moon or a planet and the sun, balance out the centripetal force experienced by a smaller body. This results in a stable or semi-stable location where the smaller body can maintain a position relative to the two larger bodies, effectively "parking" in that location.
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A Lagrange point is an equilibrium of the circular restricted three-body problem in the frame rotating with the two massive bodies.