"Delete" is like "dead", except that the posts aren't visible even if you have showdead turned on. We don't ever delete things unless users ask us to—typically because the window for deleting their own post has expired—and then only when they have a good reason for asking. Otherwise HN's bias is toward preserving history.

Coincidentally, I posted something else about "delete" a few hours ago: https://news.ycombinator.com/item?id=7975137.