Acronym: Name Withheld
Latin: Nomen Nescio (or Non Nominandus)
Often, this tag will be added in order to say something semi-anonymously.
This process is only semi-anonymous because there are publically-available IP records in difflogs of all edits, which can be used to help track down an author's login name.
There are complete IP records which can be used to at least track down an author's ISP information, and, in some cases, general location. While a proxy or simply changing ISPs could defeat this, those take a lot of effort and this only helps to prevent flames and/or bad users on this wiki.