Definition of illegal

Anything that causes a foul according to the rules of a game.

24 Random Essential Billiards Terms

This is playing to a higher winning score than eight in the game of one pocket.
The useless but common practice of contorting one's body while a shot is in play, usually in the direction one wishes a ball or balls to travel, as if in the vain hope that this will influence the balls' trajectories; the term is considered humorous.
Any one of numerous acts which unethical players employ to rattle or upset their opponent. Taking, making noise, and chalking your cue while your opponent is shooting are all considered sharking tactics.
A 7 inch (17.8 cm) square box drawn on a balkline table from the termination of a balkline with the rail, thus defining a restricted space in which only 3 points may be scored before one ball must be driven from the area. It developed to curtail the effectiveness of the chuck nurse, which in turn had been invented to thwart the effectiveness of the Parker's box in stopping long, repetitive runs using the anchor nurse.
The ACS Canadian affiliate organization, the Canadian Cue Sport Association.
The full fifteen ball set of pool or snooker object balls after being racked, before the break shot (i.e., same as rack, definition 2, and triangle, defn. 2). Chiefly British today, but also an American usage ca. World War I.
Also spot-stroke, spot hazard. A form of nurse shot in English billiards, in which the red ball, which must be spotted to a specific location after every time it is potted before another shot is taken, is potted in such as way as to leave the cue ball in position to repeat the same shot, permitting a skilled player to rack up many points in a single break (series of shots in one visit).
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.
A ball positioned near a pocket so that a particularly positioned object ball shot at that pocket will likely go in off it, even if aimed so imperfectly that if the warrior was absent, the shot would likely result in a miss. Usually arises when a ball is being banked to a pocket.

1- Shortened phrase of "ball-in-hand".

2 - In snooker, the ability to place the cue ball anywhere inside the boundaries of the D. This occurs at the start of a frame, and after the cue ball has been potted or forced off the table.
Also a short form of "Ball In Hand".

A short, jabbed draw stroke usually employed so as to not commit a foul (i.e. due to following through to a double hit) when the cue ball is very near to the target object ball.
This is the portion of the joint that actually connects the two sides of the cue, often called the pin or male end. This comes in a number of different sizes and shapes which some believe has an influence on the hit of the cue stick.
A highly abrasive tip tool used to shape an unreasonably flat new cue tip, or misshapen old one, into a more usable, consistently curved profile, most commonly the curvature of a nickel or dime (or equivalently sized non-US/Canadian coin) for larger and smaller pool tips, respectively. Similar to a scuffer, but deeper and rougher.
A type of nurse used in carom billiards games. With one object ball frozen to a cushion and the second object ball just slightly away from the rail, the cue ball is gently rebounded across the face of both balls, freezing the away ball to the rail and moving the frozen ball away the same distance its partner was previously, resulting in an identical but reversed configuration, in position to be struck again by the cue ball from the opposite side.
This is to intentionally foul by slightly moving the ball, or playing another type of illegally defensive shot in a game where the ball is just turned over to the other player. Like in one pocket, you still take the foul, but can leave the other player with a challenging shot.
A natural is an easy shot requiring no side spin. A shot is said to be natural if it does not require adjustments, such as a cut angle, side spin, or unusual force. A natural bank shot, for example, is one in which simply shooting straight into the object ball at medium speed and with no spin will send the object ball directly into the target pocket on the other side of the table.
Pocket openings that are significantly wider than are typical and thus allow shots hit with a poor degree of accuracy to be made that would not be pocketed on a table with more exacting pocket dimensions.
In snooker, the cushion opposite the top cushion and bounded by the yellow and green pockets (i.e. same as bottom cushion).
The intersection of the head string and long string, which is usually not marked on a table with a spot decal, unlike the foot spot, though some pool halls mark both spots so that racking can be done at either end of the table, and wear on the cloth from racking and breaking is more evenly distributed.
"Pocket billiards," or a game in which balls are shot into pockets.
Also highs, high balls, high ones. In eight-ball and related games, to be shooting the striped suit (group) of balls (9 through 15); "you're high balls" or "I've got the highs" ("you're high" is rare, because of the "intoxication" ambiguity). Compare stripes, yellows, big ones, overs; contrast low.
Same as centre pocket.
A soft joint-like plastic or linen base material. It lets the cue whip, putting more English on the cue ball.
With draw, as in "I shot that low left", meaning "I shot that with draw and with left english". Derives from the fact that one must hit the cue ball below it's equator, i.e. "low" on the ball, to impart draw. Contrast high.