The vexillum was a flag-like object used as a military standard by units in the Roman army. A common vexillum displayed imagery of the Roman aquila on a reddish backdrop.

A war flag, also known as a military flag, battle flag, or standard, is a variant of a national flag for use by a country's military forces when on land. The nautical equivalent is a naval ensign. Under the strictest sense of the term, few countries today currently have distinct war flags, most using a flag design that is also the state flag or general national flag for this purpose.