What's the "een" in Halloween?
early 15c., from Old French filtre and directly from Medieval Latin filtrum "felt," which was used to strain impurities from liquid, from West Germanic *filtiz (see felt (n.)). Of cigarettes, from 1908.
1570s, from Medieval Latin filtrare, from filtrum (see filter (n.)). The figurative sense is from 1830. Related: Filtered; filtering.
filter fil·ter (fĭl'tər)
A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter.
A device containing such a substance.
Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while passing others.
A translucent screen, used in both diagnostic and therapeutic radiology, that permits the passage of rays having desirable levels of energy.
A device used in spectrophotometric analysis to isolate a segment of the spectrum.
To pass a liquid or gas through a filter.
To remove by passing through a filter.
To pass through or as if through a filter.
1. (Originally Unix, now also MS-DOS) A program that processes an input data stream into an output data stream in some well-defined way, and does no I/O to anywhere else except possibly on error conditions; one designed to be used as a stage in a pipeline (see plumbing). Compare sponge.
2. (functional programming) A higher-order function which takes a predicate and a list and returns those elements of the list for which the predicate is true. In Haskell:
filter p  =  filter p (x:xs) = if p x then x : rest else rest where rest = filter p xs
See also filter promotion.