Happy 10th birthday to the SABR/Baseball-Reference Encyclopedia. Thanks to all those who have contributed and welcome to any who wish to contribute.
From BR Bullpen
This page discusses only the playing field itself. For the entire stadium, see Ballpark.
A baseball field, also called a baseball diamond, is the playing area for the game. The field is level, covered with grass (or an artificial grass-like surface) and dirt, and surrounded by a fence. Unlike most other sports venues, not all of the dimensions of a baseball field are fixed, so that different fields in the same league may be different sizes and shapes.
The center of action in a baseball game is a 90 foot square infield, in the shape of a diamond, which is the only area of the field whose dimensions are precisely specified. The corners of the square are bases. One base is marked with a five-sided home plate, where the batter stands, and the other three corners are marked with base bags labeled first base, second base, and third base in counter-clockwise order around the square. Near the center of the infield is the pitcher's mound.
The lines running from home to first base and third base are extended outward to the edges of the field and called foul lines. The area between the foul lines is fair territory, and is the area where batters are supposed to hit the ball. The fair area behind the infield is the outfield. The area outside of the foul lines is foul territory. Balls that first land in foul territory (or are touched in foul territory before landing), or that bounce or roll into foul territory before passing first or third base, are foul balls on which the runner may not reach base.
The entire field is surrounded by a fence or other barrier that marks its boundaries and serves to physically separate the playing area from non-playing areas. The area of the field close to the fence is marked with a wide warning track that helps fielders to know when they're approaching the edge of the field. Many, but not all, fences are padded to protect players who run into them. The fence ends of the foul lines are marked with tall, yellow painted foul poles to help the umpires determine whether balls pass over the fence in fair or foul territory. Outfield fences that have complex shapes are often similarly labeled with a yellow stripe at their top so that umpires can easily tell when a ball has been hit over the fence for a home run.