The Collaborative International Dictionary of English v.0.48:

Hypocycloid \Hy`po*cy"cloid\, n. [Pref. hypo- + cycloid: cf. F.
   hypocyclo["i]de.] (Geom.)
   A curve traced by a point in the circumference of a circle
   which rolls on the concave side in the fixed circle. Cf.
   Epicycloid, and Trochoid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hypocycloid
    n 1: a line generated by a point on a circle that rolls around
         inside another circle