σημεῖον σημεῖον, ου, τό, σῆμα a sign, a mark, token, Hdt., Attic a sign from the gods, an omen, Soph., Plat.: esp. of the constellations, Eur. a sign or signal to do a thing made by flags, Hdt.; αἴρειν, κατασπᾶν τὸ σ. to make or take down the signal for battle, Thuc.; τὰ σημεῖα ἤρθη the signals agreed upon were made, Thuc. an ensign or flag, on the admiralʼs ship, Hdt.; on the generalʼs tent, Xen.:—then, generally, a standard, ensign, Eur.: hence, a boundary, limit, Dem. a device upon a shield, Hdt., Eur.; upon ships, a figure-head, Ar. a signal, watchword, Thuc. in reasoning, a sign or proof, Ar., Thuc., etc.:— σημεῖον δέ: or σημεῖον γάρ: (to introduce an argument) this is a proof of it, Dem., etc.