A baseball bat is a smooth wooden or metal rod used in the game of baseball to hit the ball after the ball is thrown by the pitcher. It is no more than 2.75 inches in diameter at the thickest part and no more than 42 inches (1067 mm) in length. It typically weighs no more than 36 ounces (1 kg). The batter uses the bat two-handed to try to hit a pitched ball fair so that he may become a runner, advance bases, and ultimately score a run or help preceding runners to score.
Although using a stick to hit a ball is a somewhat simple concept, the bat is a complex object. It is carved or constructed very carefully to allow for a quick balanced swing, while providing power. The bat is divided into several regions. The "barrel" is the thick part of the bat, where the bat is meant to hit the ball. The part of the barrel best for hitting the ball with, according to construction and swinging style, is often called the "sweet spot". The end of the barrel is not part of the sweet spot, and is simply called the tip or end of the bat. The barrel narrows down, and becomes the "handle". The handle is very thin, so that batters can comfortably set the bat in their fingers. Sometimes, especially on metal bats, the handle is wrapped with a rubber or cloth "grip". Finally, next to the handle is the "knob" of the bat, a wider piece that keeps the bat from sliding out of a batter's hands.

"Lumber" is a sometimes-used slang term for a bat, especially when wielded by a particularly good batter.