CUSP

[cusp] n [L cuspis point] (1585): point, apex: as a: either horn of a crescent moon b: a fixed point on a mathematical curve at which a point tracing the curve would exactly reverse its direction of motion c: an ornamental pointed projection formed by or arising from the intersection of two arcs or foils d (1): a point on the grinding surface of a tooth (2): a fold or flap of a cardiac valve e: a point of transition (as from one astrological sign to another or one historical period to the next): turning point; also: edge, verge "on the ~ of stardom" -- cus.pate adj -- cusped adj