The spread of weapons of mass destruction to states that did not previously have them.

plural of cosmic event horizon

Absence of a horizon.

(soil science) The topmost layer of the soil horizons; topsoil

plural of Cauchy horizon

(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.

(geology, soil science) The formation of soil horizons

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

(military) The escalation of a conflict by attacking an additional geographic region.

(rare) horizontal