Nontangential limit

Summary

A function on the unit ball has a nontangential limit at a boundary point if it converges along every approach inside a cone. The cones have their vertex at the boundary point. Such cones, called Stolz angles in the plane, keep the approach away from tangential directions. Nontangential limits are the right notion of boundary value for harmonic and holomorphic functions. Fatou's theorem says that Poisson integrals and bounded holomorphic functions have them almost everywhere.