ensign

ensign, a flag, especially the national flag. The term is most often applied to the flag flown at the stern by naval vessels in commission or by merchant vessels.

The U.S. Navy’s ensign is the same as the national flag, but many other navies have distinctive naval ensigns which are "worn" by their war vessels. In the Royal Navy the ensign has a red, white, or blue ground with the Union Jack in the upper corner next to the staff. Until 1864, ships of the Royal Navy were divided into three squadrons and flew the red, white, or blue ensign to indicate the squadron to which they were assigned. Since 1864 the white ensign (further distinguished by having a red St. George’s cross quartered upon it) has been reserved for use by the Royal Navy and by the Royal Yacht Squadron. Passenger liners or other merchant vessels manned by a prescribed percentage of officers and men of the Royal Naval reserve are entitled to fly the blue ensign. Certain other vessels, not of the Royal Navy but owned by the British government, also use the blue ensign.