Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Vexillum

The Latin name for a four-cornered flag (Tertull. Apol. 16), attached to a crosspole, and carried by the vexillarius. (See Signum). Every squadron (turma), and probably every detachment of a body of troops which formed a separate command, had a red, white, or purple vexillum of this kind, and hence were themselves called a vexillum, or sometimes a vexillatio. The latter word, however, from the end of the third century A.D., signifies a squadron of cavalry. At Rome a red flag was displayed on the Capitol during the deliberations of the Comitia Centuriata, and was in time of war planted as the signal for battle on the general's tent or the admiral's ship. Vexilla served also as marks of distinction for the higher officers. See Labarum; Signum.