A baseball is the ball used in the sport of baseball. It consists of a rubber or cork center wrapped in yarn and covered with white natural horsehide or cowhide, or a synthetic composite leather. A regulation baseball is 9 to 9.25 inches (229 to 235 mm) in circumference i.e. 2.86 to 2.94 inches (73 to 75 mm) in diameter, with a weight of 5 to 5.25 ounces (0.142 to 0.149 kg).

The leather cover is commonly formed from two saddle-shaped pieces stitched together, typically with red-dyed thread. There are 108 stitches on a regulation baseball. That stitching plays a significant role in the trajectory of a thrown baseball due to the drag caused by the interaction between the stitching and the air. Controlling the orientation of the stitches and the speed of the ball's rotation allows a pitcher to affect the behavior of the pitched ball in specific ways. Commonly employed pitches include the curveball, the slider, the two-seam fastball, the four-seam fastball, the sinker, the cutter and the changeup.

In the early, mid-1800s days of baseball, there was a great variety in the size, shape, weight, and manufacturing of baseballs. Early baseballs were made with a rubber core from old, melted shoes, wrapped in yarn and leather. Pitchers usually made their own balls, which were used throughout the game, softening and coming unraveled as the game went on. One of the more popular earlier ball designs was the "lemon peel ball," named after its distinct four lines of stitching design. Lemon peel balls were darker, smaller, and weighed less than other baseballs, prompting them to travel further and bounce higher, causing very high-scoring games.

In the mid-1850s, teams in and around New York met in an attempt to standardize the baseball. They decided to regulate the weight of baseballs at 51⁄2–6 oz, and with a circumference of 8–11 inches. There were still many variations of baseballs since they were completely handmade. Balls with more rubber and a tighter winding went further and faster (known as "live balls"), and balls with less rubber and a looser winding (known as "dead balls") did not travel as far or fast. This is generally true for all baseballs. Teams often used this knowledge to their advantage, as players from the team usually manufactured their own baseballs to use in games.

There is no agreement on who invented the commonplace figure-8 stitching on baseballs. Some historians say it was invented by Ellis Drake, a shoemaker's son, to make the cover stronger and more durable. Others say it was invented by Colonel William A. Cutler and sold to William Harwood in 1858. Harwood built the nation's first baseball factory in Natick, Massachusetts, and was the first to popularize and mass-produce baseballs with the figure-8 design.

In 1876, the National League (NL) was created, and standard rules and regulations were put in place. A.G. Spalding, a well-known baseball pitcher who made his own balls, convinced the NL to adopt his ball as the official baseball for the NL. It remained that way for a century.^{[citation needed]}

In 1910, the cork-core ball was introduced. They outlasted rubber core baseballs; and for the first few years they were used, balls were hit further and faster than rubber core balls. Pitchers adapted with the use of the spitball, which is now illegal, and an emphasis on changing the ball.^{[failed verification]}

In 1920, a couple of important changes were made to baseballs. They began to be made using machine winders and a higher grade of yarn from Australia.^{[citation needed]} Offensive statistics rose immediately, and players and fans alike believed the new balls helped batters hit the ball farther.^{[citation needed]}