—A metal ring, tube, or cap placed over the end of a stick, handle, or post for added strength or stability or to increase wear.
When you hear people talking about the ferrule of a brush, they’re talking about the (usually) metal ring that connects the base of the bristles to the handle of the brush. Ferrules serve two main purposes.
First, they re-enforce the attachment of the bristles to the rest of the brush. The bristles and handle are held together by glue and the ferrule acts to protect the place where the two connect.
Second, they determine the shape and/or density of the brush. Many times, the ferrule will be pinched in the middle. Depending on how and to what degree the ferrule is pinched, the bristles will be more or less densely packed.