Definition of legal

As in many other sports, "legal" means not causing or likely to cause a foul (the opposite being illegal). A legal hit is one in which the requirements for a non-foul hit are met (e.g., in nine-ball, the lowest-numbered ball on the table was hit by the cue ball first, and at least one object ball was pocketed, or any ball reached a cushion, after the hit on the first object ball.). A legal shot is one in which no foul of any kind was involved (e.g. there was not a double hit by the cue, the player's bridge hand did not move a ball, etc.). A legal stroke is one in which the cue stroke obeyed the rules (e.g. the shooter did not perform an illegal jump shot by scooping under the cue ball with the cue tip). A legal ball is a ball-on, an object ball at which it is permissible for the player to shoot. And so on. The term can be used in many ways consistent with these examples ("legal pocket" in one-pocket, "legal equipment" under tournament specifications, etc.).

24 Random Essential Billiards Terms

This is the apex ball in the triangle, racked on the foot spot in a normal game.
This is a table that offers poor conditions for play; it is either dirty, wet, or overall poor quality.
A type of spin imparted to the cue ball to make it rebound from a cushion at a shallower angle than it would if the spin had not been used.
The break box is a zone in the "kitchen" of the head (British: bottom) of the table, from which the break shot must be taken with the cue ball,
When the cue ball is tucked behind the corner of a pocket, therefore not allowing a direct shot at the object ball without it bouncing of the corner of the rail.
The imaginary line drawn perpendicular to the impact line between the cue ball and an object ball. The cue ball will travel along this line after impact with an object ball if it has no vertical spin on it (is sliding) at the moment of impact on a non-center-to-center collision. See also stun shot.
A pool table spread in which the balls are extremely easily positioned for a run out, and where little movement of the cue ball on each shot is necessary to obtain position on the next.
This is a shot that shows great control and positioning in where the cue will be when all the balls stop rolling.
The United States Snooker Association (USSA) is the internationally recognized governing body of the sport of snooker in the United States. Founded in 1991 by the British-born Michael Collins, the USSA is a member of the International Billiards and Snooker Federation (IBSF), the world governing body of non-professional English billiards and snooker, and is affiliated with the American CueSports Alliance (ACS).
A common way to keep track of games won when playing for small money is to use a coin that is placed under the rail next to the diamonds on the rail. The center diamond at the head of the table is taken as zero, and each diamond from that is considered to be one game. To go 'around the world' is to beat your opponent so badly that the coin travels all the way around the diamonds on the table.

1- Applying very powerful follow on the cue ball thereby causing the maximum amount of follow.

2- A powerful follow shot with a high degree of top spin on it; usually when the object ball being hit is relatively close to the cue ball and is being hit very full; also known as "prograde top spin" or "prograde follow" (when referring to the action on the shot rather than the shot per se), and as a "jenny" in Australia.

The object ball involved in a key shot.
A Baulk line is line drawn across the table 29 inches from the bottom cushion and parallel to that cushion.
The point on the table surface over which the apex ball of a rack is centered (in most games). It is the point half the distance between the long rails' second diamonds from the end of the racking end of the table. The foot spot is the intersection of the foot string and the long string, and is typically marked with a cloth or paper decal on pool tables.
This is a low bet in a game with action.
Play, from the opening break shot until one player has won (or the game has been halted for some reason by a referee). Games are the units that make up matches, races (in some senses of that term) and rounds. Essentially the same as frame, except with regards to straight pool, which is a multi-rack game.
Same as angle of reflection.
Netted or cupped pockets that do not return the balls to the foot end of the table by means of a gutter system or sloped surface beneath (they must instead be retrieved manually).
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.
Verb form: to shoot. The use of the cue to perform or attempt to perform a particular motion of balls on the table, such as to pocket (pot) an object ball, to achieve a successful carom (cannon), or to play a safety.
In snooker, the second-highest value colour ball, being worth six points.
You can cheat the pocket by angling your shot to go in at the right or left of the pocket opening when sinking a shot. This permits the cue ball to strike the object ball at a different contact point than the most obvious one.
Chiefly British: The half of the table from which the break shot is taken. This usage is conceptually opposite that in North America, where this end of the table is called the head.
When an opponent either purposely successfully executes a defensive shot or misses their shot resulting in a bad leave for the next player.
Example: "I was left bad every time he missed his shot".