A filter is a type of screen that allows liquids or air to pass through while trapping sold, unwanted substances:

  • An air filter removes dirt in the air as it goes through a machine.
  • A gas filter in your car traps impurities found in gasoline.
  • A coffee filter is used in a coffee maker. The filter allows water to pass through the coffee grounds.
  • I have to change the furnace filter in my house every few months because it gets dirty.
  • A vaccum cleaner has a filter that traps very small particles of dirt and dust.
  • Saltwater must go through a filtration process in order to separate the salt from the water. (The word "filtration" is also a noun.)

coffee filters These are coffee filters.

It’s possible to use "filter" as a verb. In this case, it’s in reference to the screening or selection of people or things.

  • Applicants for jobs are filtered through the interview process.
  • The media act as a filter for stories that are fake.
  • Football tryouts filter out the players who aren’t good enough to make the team.
  • Immigrants who enter the United States are filtered though Homeland Security which checks people for criminal backgrounds.

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Date of publication: December 8, 2016