CUSP

noun Etymology: Latin ~is point Date: 1585 point , apex : as, a point of transition (as from one historical period to the next) ; turning point , either horn of a crescent moon, a fixed point on a mathematical curve at which a point tracing the curve would exactly reverse its direction of motion, an ornamental pointed projection formed by or arising from the intersection of two arcs or foils, e. a point on the grinding surface of a tooth, a fold or flap of a cardiac valve, ~ate adjective ~ed adjective