What is the ball on top of a flagpole called?

"Finial Ball"

On top of a flag pole is the flagpole truck and finial. A flagpole truck is a set of bearings that swings around with the flag and keeps it from wraping around the pole and the finial, which is normally a ball, sits on top.

On most flagpoles it is imaginatively called a ball. Flagpoles with trucks are usually used by the military. Finial is a general term for an ornamental terminating part.

A finial is an element marking the top or end of some object, often formed to be a decorative feature. In architecture it is a decorative device, typically carved in stone, employed decoratively to emphasize the apex of a dome, spire, tower, roof, or gable or any of various distinctive ornaments at the top, end, or corner of a building or structure.

https://en.wikipedia.org/wiki/Finial#Flagpoles

finial ball

A "ball-style" finial is commonly mounted to the top of a stationary flagpole. This is sometimes (mistakenly) called a "truck", which is the mechanism that connects the finial to the pole.

https://en.wikipedia.org/wiki/Finial#Flagpoles

Staff ornament or finial.

http://www.netstate.com/states/symb/flags/flag_terminology.htm

zbeckbee seems to be right, according to what I've read. I think you're looking for "truck," is the mechanism that connects the finial to the pole and may have the pulleys attached.

"A "ball-style" finial is commonly mounted to the top of a stationary flagpole. This is sometimes (mistakenly) called a "truck", which is the mechanism that connects the finial to the pole. An urban legend says the ball contains a razor, lighter, and flare. According to the legend, the items were meant to be used to destroy the flag in case the post was overrun by the enemy. A related and less pleasant version says the finial should have a bullet, knife, and matches so the last surviving soldier can continue fighting (with the knife), and if need be destroy the flag with matches (to prevent it from being captured), and take his own life with the bullet."

https://en.wikipedia.org/wiki/Finial

star_gazer replies: Finial ball