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.
National ensigns 
In nautical use, the ensign is flown at the stern of a ship or boat to indicate its nationality. Ensigns are usually flown from the stern flagpole of a ship, and may be shifted to a gaff (provided the ship is so equipped) when the ship is under way, where the ensign is known as a steaming ensign. Vexillologists distinguish three varieties of a national flag when used as an ensign:
- A civil ensign (usage symbol ) is worn by merchant and pleasure vessels. In some countries the yacht ensign, used on recreational boats or ships instead of merchant vessels, differs from the civil ensign.
- A state ensign or government ensign (usage symbol ) is worn by government vessels, such as coast guard ships.
- A naval ensign (usage symbol ) is used by a country's navy.
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.
Several Commonwealth countries' national flags had their origin in the ensigns of their original colonising power, the United Kingdom. Most notable of these national flags are those of Australia, New Zealand, and several smaller island nations, though it is also very likely that the original design from which the flag of the United States developed was strongly influenced by the British Red Ensign or the flag of the (British controlled) East India Company. In countries such as Australia and New Zealand which have national flags based on the design of British ensigns, the national flag is often (erroneously) referred to as the national ensign.
Heraldic ensigns 
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Air ensigns 
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.
- Znamierowski. p. 31. Missing or empty
- Znamierowski. "Types of flags". p. 31.
- Znamierowski. "Naval ensigns and flags". p. 88.
- Znamierowski. "Air force flags". p. 85.
- Znamierowski, Alfred (2002). The world encyclopedia of flags : The definitive guide to international flags, banners, standards and ensigns. London: Hermes House. ISBN 1-84309-042-2.