WORDWISE:

All of the figurative uses involving "ballpark" are Americanisms, and refer to a place where baseball games are played. The word is found in British English, but it is a borrowing from American usage.

The main sense of ballpark is "an acceptable, expected, or approximate range". In this sense the word is almost always found in phrases of the sort "in the ballpark" or "out of the ballpark."

A slightly later sense is "one's home territory, regarded as giving an advantage over an opponent; (hence) a field of expertise or knowledge".

Finally, ballpark is used as an adjective meaning "approximate" ("a ballpark estimate") - you will often hear this usage in business.

Here's what Merriam-Webster has to say:

(1) ball-park

Date: 1897
Function: noun
1 : a park in which ball games (as baseball) are played - in the ballpark : approximately correct


(2) ballpark

Date: 1967
Function: adjective
Etymology: from the expression in the ballpark
1 : approximately correct