Smothered mate occurs in chess when checkmate is delivered by a knight, the opposing king being unable to escape the check due to being completely hemmed in by his own pieces and (usually) the edge of the board. The knight is the only piece capable of giving smothered mate, due to its ability to jump over pieces, which gives its attack an x-ray effect.
Chess players generally find smothered mates to be among the most aesthetically pleasing checkmates, due to their economy (only one attacking piece is involved in the final position). There is a very common smothered mate theme which recurs in game after game in different positions, in which the queen is sacrificed in order to force first the opponent's king, and then his rook, into the corner, followed by the fatal knight move. In its simplest form, this checkmate is described below.
Starting position:
+----+----+----+----+----+----+----+----+
| | q | | | r | | | k |
+----+----+----+----+----+----+----+----+
| | | | | | | p | p |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | N | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | Q | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | P | P | P |
+----+----+----+----+----+----+----+----+
| | | | | | | K | |
+----+----+----+----+----+----+----+----+
Black is threatening back rank mate on e1, and is ahead on material. However, White has sacrificed in order to reach this position, and now continues:
1.Nf7+ Kg8
2.Nh6+ (The only move. Any other discovered check from the queen results in Black capturing it on b3, but this is double check from both the queen and the knight, which must be answered by a king move.)
2...Kh8 (2...Kf7 loses to 3.Qf7++)
3.Qg8+!! Rxg8 (The queen sacrifice leaves Black no choice but to capture with the rook. Note how the rook locks the Black king into the corner.)
4.Nf7++ Smothered mate. QED.
+----+----+----+----+----+----+----+----+
| | q | | | | | r | k |
+----+----+----+----+----+----+----+----+
| | | | | | N | p | p |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
| | | | | | P | P | P |
+----+----+----+----+----+----+----+----+
| | | | | | | K | |
+----+----+----+----+----+----+----+----+