| filter | (noun) device that removes something from whatever passes through it | - |
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| (noun) an electrical device that alters the frequency spectrum of signals passing through it | - |
| filth | (noun) an offensive or indecent word or phrase | Synonyms: dirty word, obscenity, smut, vulgarism |
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| (noun) a state characterized by foul or disgusting dirt and refuse | Synonyms: filthiness, foulness, nastiness |
| (noun) anything regarded as making something unclean | Synonyms: dirt, grease, grime, grunge, soil, stain |
| (noun) any substance considered disgustingly foul or unpleasant | Synonyms: crud, skank |
| filthiness | (noun) moral corruption or pollution | - |
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| (noun) a state characterized by foul or disgusting dirt and refuse | Synonyms: filth, foulness, nastiness |
| filtrate | (noun) the product of filtration; a gas or liquid that has been passed through a filter | - |
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| filtration | (noun) the act of changing a fluid by passing it through a filter | - |
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| (noun) the process whereby fluids pass through a filter or a filtering medium | - |
| infiltration | (noun) a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection) | - |
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| (noun) the slow passage of a liquid through a filtering medium | Synonyms: percolation |
| infiltrator | (noun) an intruder (as troops) with hostile intent | - |
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| (noun) someone who takes up a position surreptitiously for the purpose of espionage | - |