In the real world, a filter is a device that separates one substance from another. See also
Enumerable#filter, an alias for
In Nanoc, a filter transforms data, e.g. from Markdown to HTML.
A better name in Nanoc would be transform.
Identifiers are technically paths as of Nanoc 4 (at least when not using the legacy identifiers), but there is a distinction which is not clear.
Better names could be: