D defs.my
Entry 1 sense Webster, 1913

Keystone

/kēs'-tōnˌ/ · Key·stone · IPA /ˈkiː.stəʊn/
01 n. The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorat…
  1. 1.
    The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving. See Illust. of Arch.(Arch.) See: Arch