Because it is a type of lamp.
The ornament that screws on top of a lamp is called a "finial".
It was called the T. A. Edison electric - lamp. In Europe it was called the Ediswan lamp.
An adult monkey is called an adult monkey
it is a kid
lamp
moon
If you are talking about the support that holds the lampshade to the lamp base it is called a "lamp harp".
Hatchling -> baby -> juvi -> adult. An adult leopard gecko would be called an adult.
The adult seahorse is called seahorse.
An adult zebra is called a zebra.
An adult rat is called a rat.