noun 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.
noun Electronics or software that separates unwanted signals (for example noise) from wanted signals or that attenuates selected frequencies.
noun Any item, mechanism, device, or procedure that acts to separate or isolate.
He runs an email filter to catch the junk mail.
noun Self-restraint in speech.
He's got no filter, and he's always offending people as a result.
noun 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.
noun 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.
noun An appearance-altering digital image effect.
verb To sort, sift, or isolate.
This strainer should filter out the large particles.