Source : Webster's Revised Unabridged Dictionary (1913)

Cusp \Cusp\ (k?sp), n. [L. cuspis, -idis, point, pointed end.]
   1. (Arch.) A triangular protection from the intrados of an
      arch, or from an inner curve of tracery.

   2. (Astrol.) The beginning or first entrance of any house in
      the calculations of nativities, etc.

   3. (Astron) The point or horn of the crescent moon or other
      crescent-shaped luminary.

   4. (Math.) A multiple point of a curve at which two or more
      branches of the curve have a common tangent.

   5. (Anat.) A prominence or point, especially on the crown of
      a tooth.

   6. (Bot.) A sharp and rigid point.

Cusp \Cusp\, v. t. [imp. & p. p. {Cusped} (k?spt); p. pr. & vb.
   n. {Cusping}.]
   To furnish with a cusp or cusps.

Source : WordNet®

cusp
     n 1: point formed by two intersecting arcs (as from the intrados
          of a Gothic arch)
     2: a thin triangular flap of a heart valve [syn: {leaflet}]
     3: small elevation on the grinding surface of a tooth