A cylinder, usually zirconia ceramic, with a center hole bored to about 125 µm to hold the stripped fiber in epoxy or by mechanical clamp. The end is polished together with the fiber to form the connector end face. Ferrules come in 2.5 mm diameter for SC, ST and FC connectors and 1.25 mm for LC, and they align inside the split sleeve of an adapter.
There's epoxy bead on the ferrule tip, take it down on the 5 micron film before you even think about the fine paper.
Related terms
Connector
A removable termination that aligns the end of one fiber with another so light passes between them.
End face
The polished tip of a connector ferrule and fiber, where light leaves one fiber and enters the next.
Barrel
Installer slang for a fiber adapter, the double-ended bulkhead part that joins two connectors.
Epoxy and polish
The field termination method of gluing fiber into a connector and polishing the end face by hand.