What is the term for a short piece of line used to hold tension on a mooring?

The term for a short piece of line used to hold tension on a mooring is a "stopper." This specific function is crucial in marine operations, as it helps to secure and manage the mooring lines effectively, maintaining appropriate tension without letting the lines slip or come loose.

A stopper is typically used in scenarios where maintaining tightness is essential, such as when first securing a vessel to a docking facility or during specific maneuvers, ensuring that the lines remain under the correct amount of strain.

In contrast, other options refer to different types of lines or their uses. A leader usually denotes a line attached to a fishing tackle or similar setup, while a tag line is generally used to assist in guiding or controlling an object, often when lifting or moving heavy items. A hitch, on the other hand, is a method of securing a line but does not specifically refer to a short piece for tensioning. Thus, in the context of the question, "stopper" accurately describes the tool employed to maintain tension on a mooring line.