cusp

1. A sharp point or pointed end.
2. An important moment when a decision is made that will determine future events. (figuratively)
3. A point of a curve where the curve is continuous but has no derivative, but such that it has a derivative at every nearby point.
4. A point made by the intersection of two curved lines or curved structures, a common motif in Gothic architecture.
5. A boundary between zodiacal signs and houses.
6. Any of the pointed parts of a canine tooth or molar.
7. A flap of a valve of a heart or blood vessel.
8. A point of transition.

الجمع cusps