Pin
A piece that cannot move without exposing something more valuable behind it. Add attackers before it escapes.
White to play
Bg5
The knight on f6 now cannot move without exposing the queen sitting behind it.
What to look for
Look along your bishops', rooks' and queen's lines for two enemy pieces in a row, with the more valuable one behind.
When it applies
Once a piece is pinned, stop looking for a way to take it and start looking for a second attacker. A pinned piece cannot run, so you can build the attack at leisure — usually with a pawn.
How it goes wrong
A relative pin is not absolute. If the piece behind is a queen rather than a king, they may simply move it and let you have the exchange.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.