A metal screen made of brass or steel that is placed towards the bottom of a tobacco pipe about 1/8 inch from the bottom. It is used to prevent Dottle from forming and being drawn into the stem filter or into the smokers mouth.
Dottle is truly disgusting and for pipe smokers that "Push" (push saliva down the stem into the bowl accidentally) dottle is unavoidable without using a screen.
John used to avoid smoking black cavendish because it's sweet flavor would cause him to push, and the resulting dottle would ruin his pipe forever. Now that John bought a pack of pipe screens he can finally smoke black cavendish without forming dottle!