tro·choid 
(trōkoid′, trŏkoid′)
(trōkoid′, trŏkoid′)n.
A curve traced by a point on or connected with a circle as the circle rolls along a fixed straight line.
adj. also tro·choi·dal (trō-koidl, trŏk-oidl)
1. Capable of or exhibiting rotation about a central axis.
2. Permitting rotation, as a pulley or pivot.
[Greek trokhoeidēs, wheellike : trokhos, wheel; see TROCHEE + -oeidēs, -oid.]
tro·choidal·ly adv.