According to the USGA the definition of the teeing ground is: “The starting place for the hole to be played. It is a rectangular area two club-lengths in depth, the front and the sides of which are defined by the outside limits of two tee-markers. A ball is outside the teeing ground when all of it lies outside the teeing ground. When a player is putting a ball into play from the teeing ground, it must be played from within the teeing ground and from the surface of the ground or from a conforming tee in or on the surface of the ground.”
Although the ball has to be inside the teeing ground you may stand outside the teeing ground while striking the ball that is inside the teeing ground. If you play a ball from outside the teeing ground, you incur a penalty of two strokes and must re-tee a ball from within the teeing ground. So next time you are ready to tee off, make sure your ball is in the teeing ground!