Why is it called a singlet?

When I first heard the term "singlet" in running, I thought of this.

Source

Instead, they were talking about this.

It took me about a year to gain this little nugget of runner wisdom -- not before I read the whole book Once a Runner with the image of the first one in my mind. The entire book. I was confused and knew it would be really uncomfy (the wrestling singlet) but thought it was just a runner thing. Big difference, runners. 

By the way, what happened to just calling it a tank top?