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filter

/ˈfɪltər/·verb

to pass something through a barrier that holds back the unwanted part
Fig. 1 — A murky pour comes down the funnel with its grit riding along.
01Definition

To filter is to pass something through a barrier that holds back what should not continue — grit from water, spam from an inbox, noise from a signal. The word comes from medieval Latin filtrum, 'felt', because liquids were once strained through felt cloth. What the barrier catches is the residue; what it lets through is, in a lab, the filtrate. The modern uses stay loyal to that picture: you filter out the unwanted and keep the clear part, whether the pour is water, light, or information.

02In use
  • iThe plant filters river water through sand and charcoal before anyone drinks it.
  • iiAnalysts filter the raw readings first, setting aside any anomaly before they aggregate the results.
  • iiiDark curtains filter the afternoon light down to a soft glow.
03Collocations
  • filter out
  • filter through
  • a water filter
  • filter the results
  • a filter bubble

Family filtration (noun) · filtered (adjective) · filtrate (noun)

04Relations

=sift, strain, screen, purify, sieve

≠contaminate, mix

06TOEFL & IELTS

In integrated tasks, 'filter out irrelevant information' is the study-skills collocation both exams smile on, and process diagrams (water treatment is an IELTS Task 1 classic) need the verb plus its nouns: filtration for the process, filtrate for what passes, residue for what is caught. The phrasal filter out is separable — filter the noise out — while filter through flips the viewpoint to what gets past. Noun and verb are identical: a filter filters.

07Asked
What does 'filter out' mean?
To remove the unwanted part of something as it passes: sunscreen filters out UV rays, spam software filters out junk, a careful reader filters out the noise. Its near-synonyms carry different pictures — weed out suggests hunting bad items down one by one, screen out suggests a checkpoint with criteria, while filter out is the most passive of the three: the barrier simply sits there, and the unwanted stops itself.
What does it mean when someone 'has no filter'?
That they say whatever they think, with no strain between brain and mouth — blunt, sometimes refreshingly honest, sometimes tactless. The metaphor is the mesh in the scene above: most speakers hold something back, and this person's barrier is missing. The hashtag sense (#nofilter) is different — a photo posted without editing effects — though both trade on the same idea of nothing intercepted.
What is a filter on social media?
A software overlay that alters a photo or video in real time — smoothing skin, adding dog ears, shifting colors. Note the quiet inversion: a physical filter removes something from what passes through, while a selfie filter mostly adds. The naming still makes sense from the camera's side — the effect sits between lens and image, changing what comes out the far end.
What is a filter bubble?
The situation created when recommendation algorithms keep serving you content that matches what you already clicked, quietly straining out contrary views — a term coined by activist Eli Pariser and popularized in his 2011 book The Filter Bubble. You end up inside a bubble of agreeable information without having chosen it. The phrase is now standard in media-literacy and exam-style discussions of technology's effect on public debate.
Is filter a noun or a verb?
Both, unchanged: a coffee filter filters coffee. The verb is usually transitive (filter the water) but has a quieter intransitive life for gradual movement — 'guests filtered into the hall', 'light filtered through the blinds' — where whatever moves behaves like liquid finding its way through a barrier. That movement sense turns up in fiction and news prose more than learners expect.
What is the difference between filtration, filtrate, and residue?
Filtration is the process, the filtrate is the liquid that made it through the barrier, and the residue is what the barrier caught — in the scene above, the clear glassful and the dark layer on the mesh, respectively. The trio earns its keep in IELTS process diagrams, where water treatment is a classic subject: sedimentation lets the heavy particles settle first, filtration catches what remains, and the arrow labels are exactly these nouns.