brachistochrone
A cycloid; the curve of fastest descent between two points.
Let A and B be two fixed points. Then the time it takes a particle to slide under the influence of gravity along some path joining A and B depends on the choice of the path (curve), and hence is a functional. The curve such that the particle takes the least time to go from A to B is called the brachistochrone.
Where it comes from
From Ancient Greek βράχιστος (brákhistos, “shortest”) + χρόνος (khrónos, “time”). The term was coined by Johann Bernoulli.
Adapted from the Wiktionary entry brachistochrone (CC BY-SA 4.0).