one of five points in the orbital plane of two bodies orbiting about their common center of gravity at which another body of small mass can be in equilibrium.

British Dictionary definitions for lagrangian point

Lagrangian point

noun

astronomyone of five points in the plane of revolution of two bodies in orbit around their common centre of gravity, at which a third body of negligible mass can remain in equilibrium with respect to the other two bodies

A point in space where a small body with negligible mass under the gravitational influence of two large bodies will remain at rest relative to the larger ones. In a system consisting of two large bodies (such as the Sun-Earth system or the Moon-Earth system), there are five Lagrangian points (L1 through L5). Knowledge of these points is useful in deciding where to position orbiting bodies.