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A pin is a line attack that freezes a piece because moving it would expose a more valuable piece behind it.

A fork is one move that attacks two or more enemy pieces at the same time, so only one of them can be saved.

A skewer is a line attack through a valuable piece, so that when it steps aside a piece behind it can be taken.

A discovered attack happens when you move one piece out of the way and uncover an attack from a bishop, rook, or queen behind it.

A discovered check is a discovered attack that puts the enemy king in check.

A double check is a check given by two pieces at once, usually a discovered check plus a check from the piece that moved.

A piece is overloaded when it has to defend two things at once and cannot cover both after you attack one of them.

Deflection is forcing a defending piece off the square or line where it was doing useful work.

A decoy is a forcing move that lures a piece onto a square where it can be exploited.

Removal of the guard is capturing or driving off the piece that defends a target, so the target can be taken.

A zwischenzug is an in-between move that you play before the recapture or reply the position seemed to require.

A sacrifice is giving up material on purpose to gain an attack, a tactic, or a better position.

Attraction is forcing a piece, often the king, onto a specific square so a follow-up tactic works.

Interference is putting a piece on the line between a defender and what it defends, so the defense cannot get through.

An x-ray is an attack or defense that operates through an enemy piece along a file, rank, or diagonal.

A windmill is a repeating discovered check, usually a rook swinging off a bishop’s diagonal, that picks up material each time.

A back-rank mate is checkmate on the eighth (or first) rank when the king is trapped behind its own pawns.

A smothered mate is checkmate by a knight when the king’s own pieces take away all of its flight squares.

Scholar's mate is a four-move checkmate that aims the queen and bishop at the weak f7 pawn (or f2 for Black).

Development is bringing pieces off the back rank to squares where they control the center and can join the fight.

King safety is how well the king is sheltered from checks and mating nets, usually by castling and a sound pawn shield.

Center control is occupying or attacking the d4, d5, e4, and e5 squares so your pieces have more room than the opponent’s.

A passed pawn is a pawn with no enemy pawn in front of it or on an adjacent file, so nothing can step in its way.

An isolated pawn has no friendly pawn on either neighboring file, so it cannot be protected by another pawn.

Doubled pawns are two friendly pawns stacked on the same file, which makes them less mobile and often easier to attack.

A backward pawn cannot be guarded by neighboring pawns and cannot safely advance because the square in front of it is controlled by the opponent.

An outpost is a square, usually in the opponent’s half, that a piece can occupy without being driven off by a pawn.

An open file is a file with no pawns on it, and it is the natural home for rooks.

A rook on the seventh rank (the opponent’s second) attacks pawns from the side and can trap the king on the back rank.

A pawn break is a pawn advance that challenges the opponent’s pawn chain and opens files or diagonals.

A fianchetto is developing a bishop to b2 or g2 (or b7 or g7) after moving the knight’s pawn one square.

Space is the territory your pawns and pieces control; the side with more space can shift pieces more freely.

Prophylaxis is a move that stops the opponent’s idea before it happens, rather than chasing a threat of your own.

A hanging piece is an unprotected unit that can be taken for free, or for more than it is worth.

Zugzwang is a position where any move you play makes your situation worse, but you still have to move.

Opposition is a relationship of the two kings, one square apart on a line, where the side that does not have to move can keep the other king out.

Triangulation is a king maneuver that takes two moves to return to a neighboring square, losing a tempo so the opponent is the one who has to give way.

The Lucena position is a winning rook endgame with a pawn on the seventh rank, decided by building a bridge for the king.

The Philidor position is a drawing method in a rook endgame: the defending rook stays on the sixth rank until the pawn reaches it, then checks from behind.

The wrong bishop is a bishop that does not control the promotion square of a rook pawn, so the defender can draw by sitting in the corner.