What was the topmost capstone of a pyramid called?

The topmost capstone of a pyramid was called a pyramidion.

A pyramidion was the small pyramidal stone placed at the summit of an Egyptian pyramid. Its pointed form completed the monument’s four sloping faces and symbolically connected the tomb to the sun god’s rays. Pyramidia—the plural form—were also used as the upper sections of obelisks.

Ancient Egyptian religious ideas associated the pyramid’s shape with the benben, a sacred mound linked to the creation of the world and the rising sun. This explains why “benben” sometimes appears as a tempting alternative answer. Strictly speaking, however, benben refers to the sacred object or concept, while pyramidion is the architectural term for the capstone.

Pyramidia were made from materials including limestone, sandstone, basalt and granite. Some were coated with copper, gold or electrum so that the summit could shine in sunlight. From the Middle Kingdom onward, surviving examples were sometimes decorated with royal names, religious symbols or scenes of solar worship. Most pyramidia from the Old Kingdom have disappeared, leaving uncertainty about the original summit stones of the Giza pyramids.