Tractrix
Century Dictionary and Cyclopedia

 n tractrix A transcendental curve invented by Christian Huygens (1629–95), the property of which is that the distances along the different tangents from the points of contact to the intersections of a certain line are all equal. It is the evolute of the catenary. The definition above given is that now usual, and implies four branches, as shown in the figure. But the original definition is that it is the locus of the center of gyration of a rod of which the end is drawn along a straight line, without any effect of momentum. So defined, the curve is confined to one side of the asymptote, and so it is usually drawn. Also tractatrix. Compare cut under syntractrix.
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Webster's Revised Unabridged Dictionary
NL. See Tractor
In science:
Other embeddings of the pseudosphere in the 3dimensional Euclidean space are also available, for example it can be regarded as the 2dimensional surface generated by the tractrix , but they are not global.
On the SO(2,1) symmetry in General Relativity
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