Focal point of a hypersurface
= Focal point of a hypersurface
A focal point of a hypersurface along an orthogonal geodesic is a point where a nonzero normal <Jacobi field> arising from variations of the initial point vanishes. Beyond the first focal point, that geodesic no longer locally maximizes proper time from a spacelike initial hypersurface.
= Focal point
{synonym}