I. ˈsī-ˌklȯid noun
Etymology: French cycloïde, from Greek kykloeidēs circular, from kyklos
Date: 1661
: a curve that is generated by a point on the circumference of a circle as it rolls along a straight line
• cy·cloi·dal sī-ˈklȯi-d ə l adjective
II. adjective
Date: 1847
1. : smooth with concentric lines of growth
cycloid scales
also : having or consisting of cycloid scales
2. : characterized by alternating high and low moods
a cycloid personality