Hypocycloids

Part of speech: noun

Definitions

1. A type of mathematical curve formed by the path traced by a point on a smaller circle rolling inside a larger circle
2. Similarly, these curves can describe specific shapes used in engineering and design applications
3. In geometry, they illustrate complex relationships between curves and are important in the study of cycloidal motion

Etymology: The word is the plural of "hypocycloid", and the singular is where the history lies. A hypocycloid is the curve traced by a point on a small circle as it rolls around the inside of a larger fixed circle, so the prefix "hypo-" is literal: the rolling happens below, or inside, the outer boundary. The prefix comes from Ancient Greek "hupo", meaning "under" or "below", which in turn descends from an older Indo-European root shared by many English words of Greek origin. The second half is "cycloid", a term for curves generated by a rolling circle. Its own ancestry is Greek too, from "kuklos", meaning "circle", so the whole word is a compound of two classical building blocks put together to name a precise mathematical object. Such terms were coined by mathematicians rather than handed down through ordinary speech, which explains why the word reads so transparently: each part names exactly what the figure does.