Baseball is a sport played between two teams usually of nine players each. It is a bat-and-ball game in which a pitcher throws (pitches) a hard, fist-sized, leather-covered ball toward a batter on the opposing team. The batter attempts to hit the baseball with a tapered cylindrical bat, made of wood (as required in professional baseball) or a variety of other materials (as allowed in many nonprofessional games). A team scores runs only when batting, by advancing its players--primarily via hits, walks, and the opposition team's fielding errors--counterclockwise past a series of three markers called bases and touching home plate arranged at the corners of a ninety-foot square, or "diamond." The game, played without time restriction, is structured around nine segments called innings. In each inning, both teams are given the opportunity to bat and score runs; a team's half-inning ends when three outs are recorded against that team.

Baseball on the professional, amateur, and youth levels is popular in North America, Central America, parts of South America, parts of the Caribbean, and East Asia. The modern version of the game developed in North America beginning in the eighteenth century. The consensus of historians is that it evolved from earlier bat-and-ball games, such as rounders, brought to the continent by British and Irish immigrants. By the late nineteenth century, baseball was widely recognized as the national sport of the United States. The game is sometimes referred to as hardball in contrast to the very similar game of softball.

In North America, professional Major League Baseball teams are divided into the National League (NL) and American League (AL). Each league has three divisions: East, West, and Central. Every year, the champion of Major League Baseball is determined by playoffs culminating in the World Series. Four teams make the playoffs from each league: the three regular season division winners, plus one wild card team. The wild card is the team with the best record among the non–division winners in the league. In the National League, the pitcher is required to bat, per the traditional rules. In the American League, there is a tenth player, a designated hitter, who bats for the pitcher. Each major league team has a "farm system" of minor league teams at various levels. These teams allow younger players to develop as players gain on-field experience against opponents with similar levels of skill.

The distinct evolution of baseball from among the various bat-and-ball games is difficult to trace with precision. While there has been general agreement that modern baseball is a North American development from the older game rounders, the 2006 book Baseball Before We Knew It: A Search for the Roots of the Game, by David Block, argues against that notion. Several references to "baseball" and "bat-and-ball" have been found in British and American documents of the early eighteenth century. The earliest known description is in a 1744 British publication, A Little Pretty Pocket-Book, by John Newbery. It contains a wood-cut illustration of boys playing "base-ball," showing a set-up roughly similar to the modern game, and a rhymed description of the sport. The earliest known unambiguous American discussion of "baseball" was published in a 1791 Pittsfield, Massachusetts, town bylaw that prohibited the playing of the game within 80 yards of the town's new meeting house. The English novelist Jane Austen made a reference to children playing "base-ball" on a village green in her book Northanger Abbey, which was written between 1798 and 1803 (though not published until 1818).

The first full documentation of a baseball game in North America is Dr. Adam Ford's contemporary description of a game that took place in 1838 on June 4 (Militia Muster Day) in Beachville, Ontario, Canada; this report was related in an 1886 edition of Sporting Life magazine in a letter by former St. Marys, Ontario, resident Dr. Matthew Harris. In 1845, Alexander Cartwright of New York City led the codification of an early list of rules (the so-called Knickerbocker Rules), from which today's have evolved. He had also initiated the replacement of the soft ball used in rounders with a smaller hard ball.[4] While there are reports of Cartwright's club, the New York Knickerbockers, playing games in 1845, the game now recognized as the first in U.S. history to be officially recorded took place on June 19, 1846, in Hoboken, New Jersey, with the "New York Nine" defeating the Knickerbockers, 23–1, in four innings.

Semiprofessional baseball started in the United States in the 1860s; in 1869, the first fully professional baseball club, the Cincinnati Red Stockings, was formed and went undefeated against a schedule of semipro and amateur teams. By the following decade, American newspapers were referring to baseball as the "National Pastime" or "National Game." The first attempt at forming a "major league" was the National Association, which lasted from 1871 to 1875. The "major league" status of the NA is in dispute among present-day baseball historians, and Major League Baseball does not include the NA among the major leagues. The National League, which still exists, was founded in 1876 in response to the NA's shortcomings. Several other major leagues formed and failed, but the American League, which evolved from the minor Western League (1893) and was established in 1901 as a major league, succeeded. The two leagues were initially rivals that actively fought for the best players, often disregarding one another's contracts and engaging in bitter legal disputes. A modicum of peace was established in 1903, and the World Series was inaugurated that fall. The next year, however, the National League champion New York Giants did not participate as their manager, John McGraw, refused to recognize the major league status of the American League and its champion, the Boston Americans. The following year, McGraw relented and the Giants played the Philadelphia Athletics in the World Series.

Compared with the present day, games in the early part of the 20th century were lower scoring and pitchers were more successful. The "inside game", whose nature was to "scratch for runs", was played more violently and aggressively than it is today. Ty Cobb said of his era especially, "Baseball is something like a war!" This period, which has since become known as the "dead-ball era", ended in the 1920s with several rule changes that gave advantages to hitters and the rise of the legendary baseball player Babe Ruth, who showed the world what power hitting could produce, altering the nature of the game. Two of the changes introduced were a move to bring the outfield fences closer to the infield in the largest parks, and an introduction of extremely strict rules governing the size, shape and construction of the ball, causing it to travel farther when hit; the aggregate result of these two changes was to enable batters to hit many more home runs.

In 1884, African American Moses Walker (and, briefly, his brother Welday) had played for the Toledo Blue Stockings of the major league American Association. An injury ended Walker's major league career, and by the early 1890s, a "gentlemen's agreement" in the form of the baseball color line effectively barred African-American players from the majors and their affiliated minor leagues, resulting in the formation of several Negro Leagues. The first crack in the agreement occurred in 1946, when Jackie Robinson was signed by the National League's Brooklyn Dodgers and began playing for their minor league team in Montreal. Finally, in 1947, the major leagues' color barrier was broken when Robinson debuted with the Dodgers. Although the transformation was not instantaneous, baseball has since become fully integrated.

Major League baseball finally made it to the West Coast of the United States in 1958, when the Brooklyn Dodgers and New York Giants relocated to Los Angeles and San Francisco respectively. The first American League team on the West Coast was the Los Angeles Angels, who were founded as an expansion team in 1961.

Baseball is unique among American sports in several ways. This uniqueness is a large part of its longstanding appeal and strong association with the American psyche. The philosopher Morris Raphael Cohen described baseball as a national religion. Many Americans believe that baseball is the ultimate combination of skill, timing, athleticism, and strategy. In this, baseball is similar to its cousin game cricket: in many Commonwealth nations, cricket and the culture surrounding it hold a similar place and affection to baseball's role in American culture.

The allure of baseball is in its subtleties: situational defense, pitch location, pitch sequence, base running, batting strategies, statistics, ballparks, history, and player personalities. It's been noted that the game itself has no time limit, and its playing surface, rather than rigidly rectangular and standardized, extends theoretically to eternity from a single point (home plate) to beyond its own fences. For the avid fan, the game—even during its slowest points—is never boring because of these nuances. Therefore, a full appreciation of baseball naturally requires some knowledge of the rules; it also requires deep observation of those endearing and enduring qualities that give baseball its unique style.