An ensign is a national flag when used at sea, in vexillology, or a distinguishing token, emblem, or badge, such as a symbol of office in heraldry. The word has also given rise to the military rank of "ensign", a rank of junior officer once responsible for bearing the ensign.
The word is derived from the Latin plural insignia.
In nautical use, the ensign is flown at the stern of a ship or boat to indicate its nationality. Ensigns may also be flown from the gaff of a ship, and may be shifted to a yardarm when the ship is under way, where it is known as a steaming ensign. Vexillologists distinguish three varieties of a national flag when used as an ensign:
Many countries do not distinguish between these uses, and employ only one national flag and ensign in all cases. Others (like the United Kingdom, Italy, Russia, South Africa, Australia, New Zealand, and Japan) use different ensigns. Such ensigns are strictly regulated and indicate if the boat is a warship, a merchant vessel or a yacht, for example.
If a warship goes into battle, large versions of naval ensigns (called battle ensigns) are used.
In heraldry, an ensign is the ornament or sign, such as the crown, coronet, or mitre, borne above the charge or arms.
With the creation of independent air forces and the growth in civil aviation in the first half of the 20th century, a range of distinguishing flags and ensigns were adopted. Such ensigns may be divided into air force ensigns (such as the Royal Air Force Ensign) and civil air ensigns. Air ensigns are often light blue in colour.