What is the name for the spikes on a triceratops tail?

Stegosaurus' spiked tail is called a "thagomizer."

Way back in 1982, a famous Far Side cartoon showed a group of cavemen clustered around a picture of a Stegosaurus' tail; one of them points to the sharp spikes and says, "Now this end is called the thagomizer...after the late Thag Simmons." The Word "thagomizer" has been used by paleontologists ever since.