\Tau"to*chrone\, n. [Gr. ?, for ? ? the same + ?
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.