The Best Base Runner Ever

First of all, I have no idea if it’s “base runner” and “base running” or if it’s “baserunner” and “baserunning.”

I’ve seen it both ways, and I don’t know which is correct. The glossary on MLB.com lists the term as baserunner, one word. Merriam-Webster says it’s base runner, two words, but also shows that baserunning is fine. Dictionary.com says it can b…