cusp > define

noun

Point formed by two intersecting arcs (as from the intrados of a Gothic arch. )
A thin triangular flap of a heart valve.
- synonyms: leaflet
Small elevation on the grinding surface of a tooth. .

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

by Webster's Revised Unabridged Dictionary

Cusp is a term which indicates the at-the-money level or current coupon issuance rate for securities. It tends to be used more frequently in the mortgage backed securities business. Also, it is used in options to indicate highly sensitive derivative statistics.