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.
4. A proper subset of a poset A, which subset includes an element less than both of every pair of its members, and includes every element more than each of its members.

   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.

Verb

1. To sort, sift, or isolate.
   • This strainer should filter out the large particles.
2. To diffuse; to cause to be less concentrated or focused.
   • The leaves of the trees filtered the light.
3. To pass through a filter or to act as though passing through a filter.
   • The water filtered through the rock and soil.
4. To move slowly or gradually; to come or go a few at a time.
   • The crowd filtered into the theater.