I've found that while chess contains a wealth of easily understood vocabulary, there are some terms that juniors have difficulty understanding. Here are sixteen of them (no particular order):
Initiative is a dynamic imbalance that can be thought of as control. A player can obtain control through superior pawn structure, space, lead in development, etc. Obtaining such control is very important, because it provides the foundation for implementing our own ideas and plans, rather than responding to those of our opponent.
Deflection and decoy are very similar tactics involving the concept of distraction. In deflection, one piece is distracted from defending another piece or square. In decoy, the distracted piece becomes the target of another tactic, such as a fork or mating attack. Deflections and decoys often involve sacrifices.
"Zugzwang" is a German word meaning "compulsion to move". In other words, a player must make a move, but has no good options. This usually results in an unpreventable material gain or checkmate. (http://en.wikipedia.org/wiki/Zugzwang)
4. en passant
Please refer to: http://chess.about.com/od/rulesofchess/ss/Specialrules_3.htm
A perpetual is an ongoing repetition, much like an infinite loop in computer programming. Since a chess game cannot go on forever, rules have been set to produce an immediate draw if a position repeats three times in the same game.
A stalemate is another type of draw, very similar to a checkmate, except lacking the check. A stalemate can be called if 1) one player has no legal moves, 2) it is said player's turn to move, and 3) the king is not in check.
A good sacrifice, often used in tactics, is like an investment. A player gives up some material (for instance, trading a rook for a knight) in order to achieve positional gain, future material gain, or checkmate. Many of the most beautiful games in chess involve some form of sacrifice.
Another German term referring to an "in-between move". While most moves/tactics seem to follow a logical and forseen pattern, one overlooked check or attack inserted in the middle of the move sequence can produce unexpected results.
It seems common sense to immediately grab a new queen when a pawn is promoted, but in certain cases, it's actually more useful to promote to a B, N, or R. As an example, some endgames may result in a stalemate if a queen is chosen, yet a win if a rook is chosen instead.
10. forced/forcing move
I find a lot of students have trouble with this concept, because they assume only a limited number of moves are possible for their opponent, even if there are many. In order to reduce the number of possibilities, a check, strong attack, or important capture is essential to stimulate your opponent to follow certain lines. Forcing moves are extremely crucial for many tactics and imbalance involving initiative.
According to Jeremy Silman, an imbalance is a "difference in position". From the very start of the game, there is an imbalance in tempo(time), since white moves first. Other imbalances include knights vs. bishops, pawn structure, centre/space, development, etc. The use of imbalances helps dictate a player's strategy. Visit the following website for some practice: http://jeremysilman.com/chess_instrctn_trnmnt/072102_ch_ins_tour.html
A gambit is a sacrifice in the opening. Often one player will give up a pawn or two in order to get a lead in development, central pawn structure, superior activity of pieces, etc. Some famous gambits include the Queen's Gambit, King's Gambit, Evan's Gambit, Smith-Morra Gambit, among many others.
The attacking player does not always win the game. The reason for this is counterplay. Even though a player may be defending for some time, he/she must always be on the lookout for other sources of play and attack. A common example is the overextended attack, in which one player uses all his/her forces to produce an attack, and if it doesn't work, the player is left with all sorts of targets/holes for the previously defending player to start firing at.
“Many hands make light work.” - John Heywood
The same concept applies to pieces. When pieces work well together, they are more effective in both attack and defense. A battery is a great example: either rooks & queen working along the same file/rank (usually file), or bishop & queen working along the same diagonal.
In the past, players such as Steinitz and Tarrasch wrote books based on "classical theory", often focusing on play directly in the centre of the board, and strict rules about avoiding weaknesses and placing pieces. More recently (1920s), players such as Reti and Nimzowitsch started a hypermodern movement, which encouraged more creative play, including central control from the flanks and gambits. Today, both views are considered correct, and it's a matter of style to determine what balance is appropriate for the individual player. http://en.wikipedia.org/wiki/Hypermodernism_(chess)
"receptive to outside impressions or influences"
Pretty much, your position is influenced largely by your opponent, and thus you have lost the initiative. Passive positions can result from lack of planning, lack of experience (especially in chess strategy), fear, discomfort in the opening, a failed attack, ...
And now for the pronounciation everyone seems to have trouble with...
Wilhelm Steinitz was the first official world chess champion, having held the title in the late 19th century. He was one of the first advocates for a positional style of play, and battled the old school of thinking in what was known as the "Ink War". While most have heard his name, it's amusing to hear the different versions of pronounciation people try! The correct version is vil'helm shtī'nits (http://www.infoplease.com/ce6/people/A0846626.html)
Wow, this post took a long time! Hope it's informative for everyone - I'll be out for the rest of the day, so that's all folks!