A program that examines incoming data to ensure that only information within certain parameters is allowed to pass through. For example, you can filter out e-mail messages based on the sender's information or certain subject lines. A filter is also a program that accepts a certain type of data as input, transforms it in some manner, and then outputs the transformed data.
For example, a program that sorts names is a filter: It accepts names in unsorted order, sorts them, and then outputs the sorted names. Basically, it is a means of narrowing the scope of a report by specifying ranges or types of data to include or exclude. Utilities that allow you to import or export data are also sometimes called filters.