Cusp

1. A multiple point of a curve at which two or more branches of the curve have a common tangent.
2. A sharp and rigid point.
3. A triangular protection from the intrados of an arch, or from an inner curve of tracery.
4. The beginning or first entrance of any house in the calculations of nativities, etc.
5. A prominence or point, especially on the crown of a tooth.
6. To furnish with a cusp or cusps.
7. The point or horn of the crescent moon or other crescent-shaped luminary.