You can think of TiddlyWiki as a database in which the records are tiddlers. A database typically provides a way of discovering which records match a given pattern, and in ~TiddlyWiki this is done with filters.
A <<.def filter>> is a concise notation for selecting a particular [[set of tiddlers|Title Selection]], known as its <<.def "output">>. Whenever ~TiddlyWiki encounters a filter, it calculates the output. Further work can then be done with just those tiddlers, such as [[counting|CountWidget]] or [[listing|ListWidget]] them.
The following example passes a filter to the <<.mlink list-links>> macro to display a list of all tiddlers whose titles are <<.olink2 tagged tag>> with the word <<.word Filters>>:
A filter output can change as tiddlers are added and deleted in the wiki. ~TiddlyWiki recalculates on the fly, automatically updating any filter-based counts or lists as well.
* <$linkcatcher message="tm-navigate" actions=<<openAdvancedSearch>> >[[Advanced Search|$:/AdvancedSearch]]</$linkcatcher> -- has a <<.advancedsearch-tab Filter>> tab that makes it easy to experiment with filters.