Windmill
A windmill is a repeating pattern in which a rook moves away to give a discovered check, captures a piece, returns with check, and repeats, winning another enemy piece with each swing.
The windmill needs three things: a rook, a bishop or queen lined up behind it against the enemy king, and a king with almost no room. The rook sits on a square, such as g7, that blocks the long diagonal to the king on h8. Every time the rook leaves that square it uncovers a check from the bishop, and the rook can take something on the way.
Each swing has two halves. The rook captures along the seventh rank with a discovered check, and the king has to step to g8. Then the rook returns to g7 with check, protected by the bishop, and the king has to go back to h8. Because every move is a check, the defender never gets time to react, and the rook can collect a piece or pawn with every second move.
To find a windmill, look for a king stuck in a corner, a rook next to it that is protected by a bishop, and enemy pieces along the same rank. To defend, keep the king out of the corner, do not leave pieces on the rank where the rook can reach them, and consider giving up material early to break the line between the bishop and the king.
In the diagram White to move plays Rxe7+. The bishop on b2 gives discovered check, and Black must play Kg8. Then Rg7+ Kh8 and Rxd7+ Kg8 follow, each swing winning another pawn, and the knight on f8 blocks the king's only other square.
It sits close to Ladder mate. A ladder mate is a checkmate on the edge of the board in which two rooks, or a rook and a queen, take turns giving check on neighboring lines until the king runs out of room.
Compare it with Underpromotion. Underpromotion is promoting a pawn to a rook, bishop or knight instead of a queen, chosen because the lesser piece wins or draws more in the position at hand.
A neighboring idea is Vacating sacrifice. A vacating sacrifice gives up a piece so that it leaves, or is captured on, a square or line that another of your pieces needs, usually to deliver mate or win material.
To see how often "Windmill" or a pattern like it decides your own games, book a first lesson: George Li reviews your games and writes a study plan around what he finds.
Related terms
A discovered check is a check from a piece that did not move: a bishop, rook or queen is unmasked when the piece in front of it leaves the line to the king.
In a discovered attack one of your pieces steps off a line, and that move unmasks a bishop, rook or queen standing behind it, so a single turn produces two threats.
A battery is two of your pieces lined up on one file, rank or diagonal, with the rear piece backing up the front one so that both press on the same target.
A combination is a forcing sequence of moves, usually built from sacrifices and checks, in which every move leaves the opponent few or no choices and the sequence ends in a concrete gain such as mate or material.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
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A first lesson starts with your games: George reviews them and builds a study plan around what he finds.