pendentive

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Architecture A triangular section of vaulting between the rim of a dome and each adjacent pair of the arches that support it.

Century Dictionary and Cyclopedia

1. n. In architecture, one of the triangular segments of the lower part of a hemispherical dome left by the penetration of the dome by two semicircular or ogival vaults, intersecting at right angles. Upon the pendentives is supported, in place of the upper part of the dome of which they are segments, an independent dome of which the diameter is equal to that of the absent upper part of the first dome, or sometimes a lantern or a tower. The true pendentive is characteristic of Byzantine architecture, and is still commonly used in the various Oriental architectures baaed upon the style of building of the Greek empire. In it was found the solution of the problem of covering a rectangular space with a vault of circular plan. The term pendentive is often extended, but incorrectly, to any architectural device occupying the position of a true pendentive, and designed to answer the same purpose, but constructed of courses laid in horizontal beds and projecting each one beyond that below, or of a succession of arches corbeled out, or in any other manner which will meet the ease. No such device, however, can be a true pendentive, unless the structure is in both form and construction a segment of a dome.
2. Having pendentives; of the shape of or pertaining to pendentives.

Wiktionary

1. n. architecture The concave triangular sections of vaulting that provide the transition between a dome and the square base on which it is set and transfer the weight of the dome.

GNU Webster's 1913

1. n. The portion of a vault by means of which the square space in the middle of a building is brought to an octagon or circle to receive a cupola.
2. n. The part of a groined vault which is supported by, and springs from, one pier or corbel.