Geodesic deviation

Geodesic deviation refers to the phenomenon in general relativity that describes how nearby geodesics—paths followed by free-falling particles—diverge or converge due to the curvature of spacetime. In a curved spacetime, even if an object starts out on a geodesic (which is the generalization of a straight line in curved space), the path of that object may not remain parallel to the path of a nearby object over time.