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