tractrix

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]