Is it ensure, insure, or assure?
mid-13c., from Old French lanterne "lamp, lantern, light" (12c.), from Latin lanterna "lantern, lamp, torch," altered (by influence of Latin lucerna "lamp") from Greek lampter "torch," from lampein "to shine" (see lamp). Variant lanthorn (16c.-19c.) was folk etymology based on the common use of horn as a translucent cover. Lantern-jaws "hollow, long cheeks" is from a resemblance noted since at least mid-14c.
in architecture, originally an openwork timber construction placed on top of a building to admit light and allow smoke to escape. Something of this idea persists in medieval examples such as the lantern above the central octagon of Ely Cathedral (14th century). The term lantern soon came to refer to the open top story of a tower, because such a construction resembled a lamp container and because beacons were occasionally placed there.