PM's as they are more commonly called are often associated as 'private' messages. In XenForo they're known as 'personal' messages. Neither seem to reflect with the current times when you have 10+ or more people inside one of these PM's, nor the common lingo (terminology) that the next generation seems to use, which is direct messages or DM's. I assume this terminology started on Twitter and people on Facebook refer it as such as well. Recently I've found people increasingly calling it a DM on the forums I visit as well, so this 'terminology' is crossing into forums as well.