An acceptable, roughly accurate approximation, as in I know you can't tell me the exact cost; just give me a ballpark figure. This term alludes to a baseball field, which is always an enclosed space. The expression is basically an extension of the somewhat earlier in the ballpark, meaning within a reasonable range, and out of the ballpark, beyond a reasonable range. [Slang; late 1960s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.