saltire (n.)

c. 1400, sautour, an ordinary that resembles a St. Andrew's Cross on a shield or flag, consisting of a bend dexter and a bend sinister crossing each other, from Old French sautoir, sautour, literally "stirrup," and directly from Medieval Latin saltarium, from neuter of Latin saltatorius "pertaining to leaping," from salire "to leap" (see salient (adj.)). The connection between a stirrup and the diagonal cross is perhaps the two deltoid shapes that comprise the cross.