filter

(1) To select data. Filters use patterns (masks) against which all data are compared and only matching data are "passed through," hence the concept of a filter. For example, e-mail clients and servers can look for messages with text patterns that are recognized as spam and delete them. An e-mail client can be set up to filter messages and store them in separate mailboxes as a way of organizing the mail, or it can be set to alert the user when a certain type of message arrives. See Bayesian filtering and spam filter.

(2) To change data. In this usage of the term, the concept of a filter (a pass-through device) is less obvious, but nevertheless widely used. For example, a sort routine changes the sequence of data. A conversion routine (import or export filter) changes one type of data, text or graphics format into another. See image filter and parse.

(3) See packet filter.

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1.		filter - (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.		filter - (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.