n. In anat.: A median symmetrical arched formation in the brain, beneath the corpus callosum and septum lucidum, vaulting over the optic thalami and the third ventricle, and running into the floor of each lateral ventricle.n. Some other arched, vaulted, or fornicated formation: as, the fornix conjunctivœ, the vault of the conjunctiva.n. In conch.: The vaulted or excavated part of a shell under the umbo.n. The more concavo-convex one of the shells of an inequivulve bivalve, as an oyster.n. In botany, a small arching crest or appendage in the throat or tube of a corolla.n. In topical geom., a part of a surface which, in conjunction with another similar part which inseparably accompanies it, increases the cyclosis of the surface by two.