Helmet

From Brickipedia, the LEGO Wiki.

The helmets of a knight, a construction worker, a racecar driver, a fireman, and Darth Vader.

A helmet is a plastic lego piece that can be many shapes and sizes. It serves as a helmet for a minifigure, and normally it covers it's entire head. However, some only cover the top of the head, hiding the hair. Others many have parts that help insert goggles or shields. The helmet connects with the head to create the "top part".