What is the name of the ball on the top of a flagpole?

The term for the ball on the top of a flagpole is called a "finial." A finial serves both decorative and practical purposes, often symbolizing a crowning feature of the flagpole. Traditionally, it is found on the pole's apex and can vary in design, often being shaped as a sphere or other ornamental styles.

While "truck" is a term that refers to the assembly at the top of the flagpole which often includes the finial and may also accommodate pulleys, it does not specifically denote the ball itself. The choices that include "flagstaff," which refers to the pole itself, and "balltop," which is not a widely recognized term, do not accurately describe the part being referenced. Thus, the finial is the correct terminology for the part that caps a flagpole.