An approximate number; an estimate. Do you have a ballpark figure for the cost of the renovations?That's just a ballpark figure—they don't know exactly how many people will be attending the event.
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]