Union Grounds was a baseball park located in the Williamsburg section of Brooklyn, New York. The grounds opened in 1862, its inaugural match being played on May 15. It was the first baseball park enclosed entirely by a fence, thereby allowing proprietor William Cammeyer or his tenant to charge admission. This permitted paying customers to watch the games from benches in a stand while non-paying spectators could only watch from embankments outside the grounds.

A ballpark, or baseball park, is a type of sports venue where baseball is played. The playing field is divided into two field sections called the infield and the outfield. The infield is an area whose dimensions are rigidly defined in part based on the placement of bases, and the outfield is where dimensions can vary widely from ballpark to ballpark. A larger ballpark may also be called a baseball stadium because it shares characteristics of other stadiums.