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Definition: 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, email clients and servers can look for messages with text patterns that are recognized as spam and delete them. An email 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.