Definition of Illegal

Chiefly Australian: Same as a force follow shot.

In the UK, a long-distance shot played to pot a ball close to a pocket with heavy top spin, so that when the cue ball hits the cushion it bounces off but then stops due to the counteraction of the spin. It is not common in competitive play, being more of an exhibition shot.

Any game which uses a rack composed of less than 15 balls.

One-on-one game play.

A set practice routine.
In British terminology, a bank shot.

In pool, the degree to which racked balls move apart upon impact by the cue ball as a result of a break shot.
In snooker, a shot sending the cue ball into the pack of red balls and separating them (after potting the ball-on). At least one split is usually necessary in each frame, since the original triangle of reds does not allow any balls to be potted reliably.

Also double-century break. In English billiards, a break of 200-299 points (i.e. double a century). Larger multi-centuries are regularly achieved. Rare in amateur play, triple centuries are routine, and quadruples not uncommon at World Professional Billiards Championships; 2007 winner Mike Russell shot four triples in the final round alone, while of sixteen competitors, three shot quadruple centuries (one once, one twice, and Russell three times). Quintuple centuries are rare even at the professional level, with only the 494 shot by nine-time World Champion Russell (who has more such titles than any other player in history as of 2007) coming close in that event. World Champion Geet Sethi holds the world record, at a duodectuple century (and then some) of 1276 consecutive points."

A situation in which a ball strikes another ball which is close to a rail and the struck ball rebounds back into the ball it was hit by; usually but not always unintended.

Chiefly British: Same as duck, and stemming from the same obvious etymology.

When a ball is in firm contact with a cushion or another ball.

The white ball struck by the cue (and so used to strike other colored, numbered, object balls) during play.

This is a method of handicapping that designates a wild ball for a lesser player to be able to pocket at any point during a game in order to win.

Principally US: One or more sets, usually in the context of gambling. See also ahead race (a.k.a. ahead session) for a more specialized usage.
Principally British: Any of a group of pre-determined frames played in a match too long to be completed within a single day's play. A best of 19 frame match, for example, is generally played with two "sessions", the first composed of nine frames, the second of ten. This term is generally used only in the context of professional snooker, as matches at the amateur level are rarely played over more than nine frames. Longer matches can be split into three or four sessions.
In the APA and CPA Leagues, session refers to the season in which League play took place. There are three sessions in each League Year-Summer Session, Fall Session and Spring Session.

Feel generally refers to that elusive quality that makes one cue feel special or superior to another. In essence, it is the cumulative effect of all of a cues characteristics, including weight, shaft diameter, balance, grip material, length, etc. It can vary greatly from one player to another. A cue that feels great to one player does not necessarily fell good to another.

A rare and extremely difficult trick jump shot that turns into a massé upon landing. Requires very precise application of spin in addition to the precise application of ball pressure to effectuate the jump. Turn-of-the-20th-century World Balkline Champion Jacob Schaefer Sr. was known to daringly perform jump massés in competition.

The European Pocket Billard Federation is the European governing body for pocket billiards. EPBF is also one of the member organization of the WPA (World Pool Billard Association)

Three equally spaced diamonds are normally between each pocket on a pool table. On a carom table, the pockets themselves are replaced by additional diamonds. Diamonds get their name from the shape of the markings traditionally used; though many today are round, square, etc., these rail markings are still referred to as "diamonds".

The Equalizer Handicap System is the handicap system developed by the American Poolplayers Association. It makes it feasible for a beginner to have a nearly equal chance in a match against a more highly skilled player.

To intentionally miss a shot (that results in a foul) in order to create a position for the cue ball that makes it hard for the other player to execute their shot. Not to be confused with a "Safety", since a safety is a legal hit.

A pool room employee who plays with a good degree of skill.

A two-piece cue constructed to resemble a house cue, with a near-invisible wood-to-wood joint. The subterfuge often enables a hustler to temporarily fool unsuspecting fish into thinking that he or she is an unskilled banger with no regard for finesse or equipment quality. Many league players also use cheap but solid sneaky petes as their break cues.

An instance of contact between balls, usually used in the context of describing an object ball contacting another object ball (e.g. "the two ball kissed off the twelve ball"), or in snooker the cue ball making contact with a ball after the initial contact with the object ball. If the player's intention was to cause two object balls to kiss (e.g. to pocket a shot ball after a ricochet off a stationary one), it is often called a kiss shot.

Noun:
1.In pool games such as nine-ball, a specific handicap given (e.g., "what spot will you give me?").
2.In snooker, any of the six designated points on the table on which a colour ball is replaced after it has left the playing surface (usually after it has been potted).
3.An (often unmarked) point on the table, at the intersection of two strings. See foot spot, head spot, center spot for examples.
4.In UK eight ball, (when not playing with a reds-and-yellows colour ball set) any of the group of seven balls, other than the 8, that are a solid colour with just a circled number on the surface. In the US, these balls are usually referred to as solids or more colloquially as lows, littles or smalls, while British terms include dots and unders. Contrast stripes.
5.Alternate name for a table's diamonds.

Verb:
1.In pool, return an illegally pocketed object ball to the table by placement on the foot spot or as near to it as possible without moving other balls (in ways that may differ from ruleset to ruleset).
2.In snooker, to return a colour ball to its designated spot on the table. Also called re-spot.
3.In nine-ball, the giving of a handicap to the opponent where they can also win by making a ball or balls other than the 9 ball (e.g. "she spotted me the seven ball").
4.In eight-ball, one-pocket and straight pool, the giving of a handicap to the opponent where they have to make fewer balls than their opponent does.
5.In some variants of pool, to place the cue ball on the head spot or as near to it as possible inside the kitchen/baulk, after the opponent has scratched.

A shot in which the cue ball is potted after caroming off another ball. In snooker and most pool games doing this would be a fault (foul), but the move will score points in many games in which hazards (as such) apply, such as English billiards, or in the final or game point in Cowboy pool. The term derives from this hazard costing the player points in early forms of billiards.