pyramidion
/ˌpɪrəˈmɪdɪən/noun
Chiefly Architecture and Archaeol. A small pyramid; especially (a) the pyramidal apex of an obelisk; (b) the capstone of a pyramid.
Origin
Mid 18th century; earliest use found in Thomas Shaw (1694–1751), traveller. From ancient Greek πυραμίδ-, πυραμίς pyramid + -ιον, diminutive suffix.