Outpost

A piece in enemy territory, beyond the demarcation line of the board, standing on an open or half-open file. This piece is most often backed by a pawn, but sometimes by a rook or a bishop.

An outpost is a good base for launching an attack and can cause a serious weakening of the opponent's position.

This position arose in the game Botvinnik — Batuev (Leningrad, 1931):

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1. Here the outpost is the white knight on e5. Not sensing the danger, Black played
2. 17.
3. That was correct
4. 18.
5. 19.
6. 20.
7. 21.
8. 22.
9. 23.
10. 24.
11. 25.

Practice has shown that on the central files a knight makes the best outpost, while on the edge files a rook does.