This tag refers to all questions deleted from Stack Exchange sites. You can also use this tag along with [specific-question] and [meta] to ask why a specific question on Meta Stack Exchange was deleted.
The tag deleted-questions refers to all questions deleted from Stack Exchange sites. The tags specific-question and meta should also be used when referencing a specific question that has been deleted on Meta Stack Exchange. (If you want to ask about a deleted question on another site, ask on its per-site meta.)
For more information on what deletion is, see How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?