A light spar from which the ensign is flown when underway is known by what term?

The correct term for a light spar from which the ensign is flown when a vessel is underway is "gaff." A gaff traditionally refers to a horizontal spar that projects from the mast of a sailing vessel and is used to extend the upper edge of a sail, but in this context, it specifically pertains to the location on a ship where the ensign (the national flag) is displayed. When a ship is underway, the ensign must be flown in a manner that makes it visible, and using a gaff allows for effective visibility suited to maritime traditions.

In naval terminology, other terms like flagpole, mast, or staff refer to different structures or uses. A flagpole typically denotes a vertical pole used for displaying flags at a stationary location, while a mast is primarily used to support sails and rigging on a ship. A staff is more traditionally associated with a pole used for displaying flags but may not specifically denote the type of spar used while underway in the maritime context. Each of these terms serves a purpose, but in this scenario, the gaff is the appropriate answer indicating where the ensign is properly flown on a vessel when it is moving.