A smothered knight, also called a “buried knight”, is a Knight that has been trapped by enemy pawn chains. It usually cannot be captured (unlike a “trapped knight” caught in enemy territory), but it is still like playing without the Knight piece. Smothering a Knight is a game-winning advantage, almost as much as winning an extra Knight.
Diagram: Knight Smothered by Pawn Chain
Diagram: Knight Smothered in the Corner