Dictionary.net

• A curve from which another curve is described; the origin of the evolvent or involute.

1914 - Nuttall's Standard dictionary of the English language
By Nuttall, P.Austin.

• n. [Latin] A curve from which another curve, called the involute or evolvent, is described by the end of a thread gradually wound upon the former or unwound from it.

1871 - The Cabinet Dictionary of the English Language