Swivel joint

A swivel joint, variously called a hinge, pin joint, or simply a "swivel", is a joint which couples two components together while allowing them rotary motion relative to each other. On Transformers toys, a swivel must sometimes be restricted from moving freely, which can be done by friction, spring-loaded tabs in slots, or ratchets.

From Teletraan I: The Transformers Wiki, a Wikia wiki.