tautochrone

from The Century Dictionary.

• noun In mathematics, a curve line such that a heavy body descending along it by gravity will, from whatever point in the curve it begins to descend, always arrive at the lowest point in the same time. The cycloid possesses this property for a constant force with no resistance.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics 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.