A banner of arms is a flag, which has the same image as a coat of arms, i.e. the shield of a full heraldic achievement, rendered in the rectangular shape of the flag. It can thus be compared with the heraldic flag but it is not only used ceremoniously but may be used e.g. as a national flag.
The term is derived from the terminology of heraldry but mostly used in vexillology. Examples of modern national flags which are banners of arms are the flags of Austria, Iraq, and Switzerland.
The banner of arms is sometimes simply called a banner, but a banner is in a more strict sense a one of a kind personal flag of a nobleman held in battle.