What does “mdot” mean?

Reviewing an excellent slide deck about how to improve the performance of responsive web sites, I encountered the term “mdot” and was mystified.

It refers to the “m.” that often begins the URLs of mobile-only websites.

Most folks who mention it out there just use it without defining it. I just had to try a lot of searches to figure this one out, so I thought I'd try to help by telling Google explicitly.