(physics) A light-like boundary of the domain of validity of a Cauchy problem. One side of the horizon contains closed space-like geodesics and the other side contains closed time-like geodesics.

A deep in Tonga Trench, Pacific Ocean; the second deepest place on Earth, and deepest part of the Tonga Trench.

(geochronology) A stratigraphic unit of distinctive lithology with a wide geographical extent used in stratigraphic correlation.

The cosmological horizon; the edge of the observable universe.