This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. A player who is on base.

n. By extension, a player who is trying to advance from base to base.

"Regardless of what actually happens to baserunners (note that our definition of 'baserunner' excludes those rounding the bases on a home run, simply for ease of writing's sake)."

baserunning