What characteristic defines a nun buoy?

A nun buoy is specifically defined by its color scheme, which is distinctive in the maritime navigation system. These buoys are typically red in color and are used to mark the right side of a channel when approaching from the open sea. The color scheme plays a critical role in helping mariners navigate safely, as it provides visual indicators of the channel's layout.

In contrast, while nun buoys may have certain physical attributes like being cylindrical or having specific side markings, those characteristics do not uniquely define them. The height of a nun buoy can vary as well, but it is the color that provides a standard recognized across navigational practices. Therefore, the color scheme is the most defining feature of a nun buoy.