A line generated by a point on a circle that rolls around inside another circle

WordNet Word definitions in WordNet
n. a line generated by a point on a circle that rolls around inside another circle

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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 .

Wiktionary Word definitions in Wiktionary
n. (context geometry English) The locus of a point on the circumference of a circle that rolls without slipping inside the circumference of another circle.

Wikipedia Word definitions in Wikipedia
In geometry , a hypocycloid is a special plane curve generated by the trace of a fixed point on a small circle that rolls within a larger circle. It is comparable to the cycloid but instead of the circle rolling along a line, it rolls within a circle.