1 : someone who rings a bell
a bell ringer
2 : the part of a telephone that rings to signal that someone is calling
Turn off the ringer on the telephone.
3 informal : a person or animal that enters a contest illegally by using a false name, pretending not to have much skill, etc.
One of the players on the winning team was a ringer.
4 informal : someone who looks very much like another person
He's a ringer for the President.
She's a dead ringer for my cousin Julie. [=she looks exactly like my cousin Julie]