An ensign is the national flag flown by a ship to designate its nationality, typically displayed at the stern (rear) of the vessel when in port or under power, often moved to the backstay or mast while sailing. It signifies the country of registration, law compliance, and ownership.

Key Aspects of the Ensign:
Location: Flown from a staff at the stern (rear).

Purpose: Indicates nationality and, in some cases, whether the vessel is a civilian, government, or naval ship.

Courtesy Ensign: A small national flag of a foreign country flown by a visiting vessel to show respect.

Yacht Ensign: In the U.S., a specific flag (often with a circle of 13 stars and an anchor) used by pleasure yachts instead of the national flag.