Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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