The apparatus, vessels, or media, employed for filtration, are called " filters" and are technically distinguished from "strainers" by the superior fineness of their pores.

Both strainers and filters act on the same principles as the common sieve on powders; they all, in like manner, retain or hold back the coarser matter, and permit the liquid or smaller and more attenuated particles to pass through. The term "medium" (plural V media") is applied to the substance or substances through the pores of which the liquid percolates.