The Collaborative International Dictionary
Tractrix \Tract"rix\, n. [NL. See Tractor.] (Geom.) A curve such that the part of the tangent between the point of tangency and a given straight line is constant; -- so called because it was conceived as described by the motion of one end of a tangent line as the other end was drawn along the given line.
Wiktionary
n. The curve that satisfies the following property: That segment of the tangent line that lies between the point of tangency and a fixed line has length independent of the point of tangency chosen.
Wikipedia
A tractrix (from the Latin verb trahere "pull, drag"; plural: tractrices) is the curve along which an object moves, under the influence of friction, when pulled on a horizontal plane by a line segment attached to a tractor (pulling) point that moves at a right angle to the initial line between the object and the puller at an infinitesimal speed. It is therefore a curve of pursuit. It was first introduced by Claude Perrault in 1670, and later studied by Sir Isaac Newton (1676) and Christiaan Huygens (1692).
Usage examples of "tractrix".
Eliza had sat next to him and pronounced their names: the Limaçon of Pascal, the Kampyle of Eudoxus, the Conchoid of de Sluze, the Quadratrix of Hippias, the Epitrochoid, Tractrix, and the Cassinian Ovals.