A baseball field is a playing field used for baseball. Professional fields are situated inside stadiums with seating for a large number of visitors.

Diagram of a baseball field.

The starting point for much of the action on the field is home plate, which is a white rubber pentagon seventeen inches wide. Next to each of the two parallel sides is a batter's box. The point of the pentagon is at one corner of a ninety-foot square. The other three corners of the square, in counterclockwise order from home plate, are called first base, second base, and third base. Three canvas bags fifteen inches (38 cm) square mark the three bases. These three bags along with home plate form the four bases at the corners of the infield. A little-known fact about the bases is that first and third bases are entirely within the ninety-foot square, as is home plate, but second base is not. Second base is placed so that its center coincides exactly with the corner of the ninety-foot square.

The lines from home plate to first and third bases are extended infinitely and are called the foul lines. The quarter of the universe between the foul lines is fair territory; the other three-quarters of the universe is foul territory. The area in the vicinity of the square formed by the bases is called the infield; fair territory outside the infield is the outfield. Most baseball fields are enclosed with a fence that marks the outer edge of the outfield. The fence is usually set at a distance ranging from 300 to 410 feet (90 to 125 m) from home plate. Most professional and college baseball fields have a right and left foul pole. These poles are at the intersection of the foul lines and the respective ends of the outfield fence. Each pole itself is in fair territory. Any ball hitting a foul pole above the top of the outfield fence is a home run, regardless of where the ball goes after striking this pole. Foul poles are not shown on the diagram seen above. Each pole is much higher than the top of the outfield fence and serves to prove that a ball going over the fence was fair (a home run) or foul (out of play).

In the middle of the square is a low mound called the pitcher's mound. There is a rubber plate, called the pitcher's rubber, six inches (15 cm) wide and two feet (61 cm) long, on the mound, exactly sixty feet six inches (18.4 m) from home plate. This peculiar distance was set due to a clerical error. When it was decided that the distance from home plate to the pitcher's mound should be increased from 50 feet to 60 feet (15.2 to 18.3 m) from the point of home plate, the builders read the diagram's notation of 60'00" as 60'06".

A baseline is the direct route—a straight line— between two adjacent bases. The basepath is the region within three feet (0.9 meters) of the baseline. Baserunners are not required to run in this objective basepath, however; a baserunner may run wherever he wants when no play is being attempted on him. At the moment the defense begins to attempt a tag on him, his running baseline is established as a direct line from his current position to the base which he is trying for. The runner may not stray three feet away from this line in an attempt to avoid a tag; if he does, he is automatically out.

The grass line, where the dirt of the infield ends and the grass of the outfield begins, has no special significance to the rules of the game. Its only purpose is to act as a visual aid so that participants, fans, and umpires may better judge distance from the center of the diamond.