CUSP (Lat. cuspis, a spear, point) , a projecting point, or pointed end. In architecture (Fr. feuille, Ital. cuspide, Ger. Knopf e), a cusp is the point where the foliations of tracery intersect. The earliest example of a plain cusp is probably that at Pythagoras school, at Cambridge, of an ornamented cusp at Ely cathedral, where a small roll, with a rosette at the end, is formed at the termination of a cusp. In the later styles the terminations of the cusps were more richly decorated; they also sometimes terminate not only in leaves or foliages, but in rosettes, heads and other fanciful ornaments. The term " feathering " is used of the junction of the foliated cusps in window tracery, but is usually restricted to those cases where it is ornamented with foliage, etc.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)