From BR Bullpen
Fair territory refers to the area of the playing field that is located within the foul lines. It is composed of the infield - the 90-foot square that is delimitated by the three bases and home plate, and the outfield, a large area whose size and shape varies from ballpark to ballpark but that is situated between the left and right foul lines. The area beyond the outfield fence, although out of play, is also technically fair territory, since a ball hit there is a home run and not a foul ball. In practice, fair territory usually refers only to the area that is in play.
The objects that demarcate fair territory are considered a part of it. These include the three bases and home plate, the foul lines, and the foul poles. A ball hitting any of these is considered a fair ball.
The area that is in play but falls outside of fair territory, either to the other side of the foul lines or behind home plate, is called foul territory.