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"baseball stadium," 1899, from (base)ball + park (n.). Figurative sense of "acceptable range of approximation" first recorded 1954, originally in the jargon of atomic weapons scientists, perhaps originally referring to area within which a missile was expected to return to earth; the reference is to broad but reasonably predictable dimensions.
The result, according to the author's estimate, is a stockpile equivalent to one billion tons of TNT. Assuming this estimate is "in the ball park," clearly there is valid reason for urging candor on the part of our government. [Ralph E. Lapp, "Atomic Candor," in "Bulletin of the Atomic Scientists," October 1954]