拉格朗日点（Lagrangian point）又称平动点（libration points）在天体力学中是限制性三体问题的五个特解。就平面圆型三体问题，1767年数学家欧拉根据旋转的二体引力场推算出其中三个点（特解）为L₁、L₂、L₃，1772年数学家拉格朗日推算出另外两个点（特解）为L₄、L₅。例如，两个天体环绕运行，在空间中有五个位置可以放入第三个物体（质量忽略不计），并使其保持在两个天体的相应位置上。理想状态下，两个同轨道物体以相同的周期旋转，两个天体的万有引力与离心力在拉格朗日点平衡，使得第三个物体与前两个物体相对静止。

In celestial mechanics, the Lagrangian points (/ləˈɡrɑːndʒiən/; also Lagrange points, L-points, or libration points) are positions in an orbital configuration of two large bodies where a small object affected only by gravity can maintain a stable position relative to the two large bodies. The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them. There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies. The first three are on the line connecting the two large bodies and the last two, L4 and L5, each form an equilateral triangle with the two large bodies. The two latter points are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies.