evolute
noun
evo·lute ˈe-və-ˌlüt 
also ˈē-və-
: the locus of the center of curvature or the envelope of the normals of a curve
Word History
Etymology
borrowed from New Latin ēvolūta (short for līnea ēvolūta, literally, "unrolled line"), noun derivative from feminine of Latin ēvolūtus, past participle of ēvolvere "to roll out or away, unwind, unroll" — more at evolve
Note: The New Latin term was introduced by Christian huygens in Horologium oscillatorium (Paris, 1673), p. 60.
First Known Use
1704, in the meaning defined above