tautochrone

n. [ Gr. &unr_;, for &unr_; &unr_; the same + &unr_; time: cf. F. tautochrone. ] (Math.) A curved line, such that a heavy body, descending along it by the action of gravity, will always arrive at the lowest point in the same time, wherever in the curve it may begin to fall; as, an inverted cycloid with its base horizontal is a tautochrone. [ 1913 Webster ]