filter

Etymology

From Middle English filtre, from Medieval Latin filtrum (compare also Old French feutre (“felt; filter”)), from Frankish *filtir, from Proto-West Germanic *felt. See felt.

noun

1. A device which separates a suspended, dissolved, or particulate matter from a fluid, solution, or other substance; any device that separates one substance from another.
2. Electronics or software that separates unwanted signals (for example noise) from wanted signals or that attenuates selected frequencies.
3. Any item, mechanism, device or procedure that acts to separate or isolate.
   He runs an email filter to catch the junk mail.
   In America alone, people spent $170 billion on “direct marketing”—junk mail of both the physical and electronic varieties—last year. Yet of those who received unsolicited adverts through the post, only 3% bought anything as a result. If the bumf arrived electronically, the take-up rate was 0.1%. And for online adverts the “conversion” into sales was a minuscule 0.01%. That means about $165 billion was spent not on drumming up business, but on annoying people, creating landfill and cluttering spam filters. 2013-05-25, “No hiding place”, in The Economist, volume 407, number 8837, page 74
4. (figurative) self-restraint in speech.
   He's got no filter, and he's always offending people as a result.
5. (mathematics, order theory) A non-empty upper set (of a partially ordered set) which is closed under binary infima (a.k.a. meets).
   The collection of cofinite subsets of ℝ is a filter under inclusion: it includes the intersection of every pair of its members, and includes every superset of every cofinite set.
   If (1) the universal set (here, the set of natural numbers) were called a "large" set, (2) the superset of any "large" set were also a "large" set, and (3) the intersection of a pair of "large" sets were also a "large" set, then the set of all "large" sets would form a filter.
6. (photography) A translucent object placed in the light path of a camera to remove certain wavelengths (colors), or a computer program that simulates such an effect.

verb

1. (transitive) To sort, sift, or isolate.
   This strainer should filter out the large particles.
   You have probably never seen anything like this before, Mr. Toler. It is baleen, or if you prefer it, whalebone, taken from the mouth of the bowhead whale. It is used by the whale to filter its food. 1954, Alexander Alderson, chapter 5, in The Subtle Minotaur
   But fans’ emotions are no longer filtered through ticket or album sales; they’re heard directly, constantly, at all hours, on all the platforms people visit to generate and extinguish bad feelings in a never-ending cycle. 2022-10-25, Willy Staley, “The Try Guys and the Prison of Online Fame”, in The New York Times Magazine
2. (transitive) To diffuse; to cause to be less concentrated or focused.
   The leaves of the trees filtered the light.
3. (intransitive) To pass through a filter or to act as though passing through a filter.
   The water filtered through the rock and soil.
4. (intransitive) To move slowly or gradually; to come or go a few at a time.
   The crowd filtered into the theater.
5. (intransitive) To ride a motorcycle between lanes on a road
   I can skip past all the traffic on my bike by filtering.