thumb|Dutch pool player [[Niels Feijen at the 2008 European Pool Championship]]
thumb|right|A player setting a
Pool is a group of cue sports played on a billiard table. The table has six pockets along the , into which balls are shot. Of the many different pool games, the most popular include: eight-ball, blackball, nine-ball, ten-ball, seven-ball, straight pool, one-pocket, and bank pool. Eight-ball is the most frequently played discipline of pool, and it is often thought of as synonymous with "pool".
The generic term pocket billiards is sometimes also used, and favored by some pool-industry bodies, but is technically a broader classification, including games such as snooker, Russian pyramid, and kaisa, which are not referred to as pool games. There are also hybrid games combining aspects of both pool and carom billiards, such as American four-ball billiards, bottle pool, cowboy pool, and English billiards.
Etymology
thumb|Historic print depicting [[Michael Phelan (billiards)|Michael Phelan's billiard saloon in New York City, 1 January 1859.]]<!-- We need much more on the history of the game -->
The etymology of "pool" is uncertain. The Oxford English Dictionary speculates that "pool" and other games with collective stakes is derived from the French poule (literally translated "hen"), in which the poule is the collected prize, originating from jeu de la poule, a game that is thought to have been played during the Middle Ages. Supposedly, participants would put an equal amount of money into a pot and throw stones at a live chicken, and the person who successfully hit the chicken first would win the pooled money. Alternatively the term could derive from the verb to pool in the sense of combining objects or stakes. The oldest use of the word "pool" to describe a billiards-like game was made in 1797 in a Virginia newspaper. The OED defines it as generally "any of various types of billiards for two or more players" but goes on to note that the first specific meaning of "a game in which each player uses a cue ball of a distinctive colour to pocket the balls of the other player(s) in a certain order, the winner taking all the stakes submitted at the start of the contest" is now obsolete, and its other specific definitions are all for games that originate in the United States. In the British Empire for most of the nineteenth through early twentieth century, pool referred specifically to the game of life pool.
Although skittle pool is played on a pocketless carom billiards table, the term pool later stuck to all new games of pocket billiards as the sport gained in popularity in the United States,
History
With the exception of one-pocket, games typically called "pool" today are descended from two English games imported to the United States during the 19th century. The first was English billiards which became American four-ball billiards, essentially the same game but with an extra red to increase scoring opportunities. It was the most popular billiards game in the mid-19th century until dethroned by the carom game straight rail. American four-ball tournaments tried switching to carom tables in the 1870s but this did not save it from being doomed to obscurity; the last professional tournament was held in 1876. New games introduced at the turn of the 20th century include Kelly pool and eight-ball. The distinctive appearance of pool balls with their many colors and division between solid and striped balls came about by 1889. Modern coin-operated pool tables generally use one of three methods to distinguish and return the cue ball to the front of the table while the numbered balls return to an inaccessible receptacle until paid for again: the cue ball is larger and heavier than the other balls, or denser and heavier, or has a magnetic core.
Modern cue sticks are generally long for pool while cues prior to 1980 were designed for straight pool and had an average length of . By comparison, carom billiards cues are generally shorter with larger tips, and snooker cues longer with smaller tips.
Game types
Racked games
These are games descended from the early 19th century games of pyramid pool and fifteen-ball pool which required balls to be racked due to the large number of them on the table. Of the other pyramid traditions of Continental Europe, only Russian pyramid survives. Snooker, originally known as snooker's pool, also has roots in pyramid.
Rotation games
thumb|One of many correct nine-ball racks: the 1 ball at the centered over the , the 9 ball at center, the other balls placed randomly, and all balls touching.
Rotation games require players to make legal contact with the lowest numbered ball on the table or a foul is called. The earliest rotation game, originally known as 61, started off as a variant of fifteen-ball pool during the mid-nineteenth century. The name "rotation" came from how the balls were placed around the table in its unracked offshoot Chicago. 61 has spawned many variations of its own such as American rotation, nine-ball, ten-ball, and Kelly pool. Of these, nine-ball is the most popular and the predominant professional game with ten-ball as the second-most prominent.<!--Additional pre-2008 sources needed here.--> There are many local and regional tours and tournaments that are contested with nine-ball. The World Pool-Billiard Association (WPA) publishes the world standardized rules. The European professional circuit has instituted rules changes to make it more difficult to achieve a legal break shot.
The largest nine-ball tournaments are the US Open Nine-ball Championship and the WPA World Nine-ball Championship for men and women. A hotly contested event is the annual Mosconi Cup, which pits invitational European and U.S. teams against each other in one-on-one and nine-ball matches over a period of several days. The Mosconi Cup games are played under the more stringent European rules, as of 2007.