A Split Ring is a type of metal ring that has been wound in such a way that there are two distinct ends. The two full turns within the split ring allow for things to be slid onto the ring in the same manner as a key ring. When connecting two items, a split ring is very secure, as there is is no way it can easily open when you don't want it to!
This is a great choice if you want to make a keyring. Price is per piece.