'billboard' is one implementation method that can used to display them in a 3D game, but the interface/design/concept is very common in 2D games too (going back many years).
Yeah, I was referring to the "technical term" for rendering them in 3D. As an interface/design concept, the term billboard is not accurate. It wasn't clear whether the OP wanted to know how to render it, or how it should be called in a design doc., an in-game tutorial, etc
I guess this would be akin to something like WoW's scrolling combat text; damage and healing numbers popping up on screen located near their point of origin, etc. If I'm not mistaken, most games render those to their HUD, same layer of their general GUI, as two-dimensional entities. You might want to check out some of the modding communities for popular MMOs to see if they have any of their source code open so you can poke around and see how it works.
