Cusp
Webster's Dictionary [1]
(1): (n.) A multiple point of a curve at which two or more branches of the curve have a common tangent.
(2): (v. t.) To furnish with a cusp or cusps.
(3): (n.) A prominence or point, especially on the crown of a tooth.
(4): (n.) A triangular protection from the intrados of an arch, or from an inner curve of tracery.
(5): (n.) A sharp and rigid point.
(6): (n.) The beginning or first entrance of any house in the calculations of nativities, etc.
(7): (n.) The point or horn of the crescent moon or other crescent-shaped luminary.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]
(Lat. a spear-point), the projecting points forming the featherings or foliations in Gothic tracery, arches, panels, etc.; they came into use during the latter part of the Early English style, at which period they were sometimes worked with a small leaf, usually a trefoil, on the end. When first introduced, the cusps sprang from the flat under-surface or soffit of the arch, entirely independent of the mouldings, and this method was sometimes followed in decorated work; but they very soon began to be formed from the inner moulding next the soffit (usually either a splay or a hollow), and this continued to be the general practice until the expiration of Gothic architecture. Some of the richest examples may be found in Lincoln Cathedral. (See Panel).
In the Decorated and Perpendicular styles they were frequently ornamented at the ends, either with heads, leaves, or flowers, and occasionally with animals. — Parker, Gloss. of Architect. s.v. (See Foils).
