The vexillum (English pronunciation: /vɛkˈsɪləm/; plural vexilla) was a flag-like object used in the Classical Era of the Roman Empire. The word is itself a diminutive for the Latin word, velum, sail, which confirms the historical evidence (from coins and sculpture) that vexilla were literally "little sails" i.e. flag-like standards. In the vexillum the cloth was draped from a horizontal crossbar suspended from the staff; this is unlike most modern flags in which the 'hoist' of the cloth is attached directly to the vertical staff. The bearer of a vexillum was known as a vexillarius or vexillifer. Just as in the case of the regimental colors or flag of Western regiments, the vexillum was a treasured symbol of the military unit that it represented and it was closely defended in combat.

Nearly all of the present-day regions of Italy preserve the use of vexilla. Many Christian processional banners are in the vexillum form; usually these banners are termed labara (Greek: λάβαρον) after the standard adopted by the first Christian Roman emperor Constantine I which replaced the imperial eagle with the "Chi-Rho" symbol ☧.