A baseball stadium or, informally, an approximate range or rough estimate. Often used in the phrase 'ballpark figure' to mean a rough calculation.
Originally American English, combining 'ball' and 'park' to describe baseball stadiums in the early 20th century. The figurative sense of 'approximate range' emerged in the 1960s, suggesting being 'in the same ballpark' as the correct answer.
The phrase 'ballpark figure' perfectly captures how Americans think about estimation - it suggests being in the right general area even if not precisely accurate, just like a ball hit somewhere within the stadium boundaries.
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