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 filter is a concise notation for selecting a particular set of tiddlers, known as its output. Whenever TiddlyWiki encounters a filter, it calculates the output. Further work can then be done with just those tiddlers, such as counting or listing them.
The following example passes a filter to the
list-links macro to display a list of all tiddlers whose titles start with the letter H:
A filter's 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.
Advanced Search has a Filter tab that makes it easy to experiment with filters.