Outpost
A square in or near enemy territory that cannot be attacked by an enemy pawn — an ideal home for a knight or bishop.
An outpost is a square that no enemy pawn can ever drive a piece off. By definition, no enemy pawn currently controls the square, and no enemy pawn can ever be advanced or moved sideways to attack it. A piece placed there is permanent until it chooses to leave.
The classical outpost is the square in front of a backward pawn or a half-open file. A knight on d5 in front of Black’s pawn on d6, with no Black pawn on c7 or e7 that could ever advance to drive the knight away, is unassailable. The pieces around it — bishops, rooks, queens — must move; the knight stays.
Outposts are most valuable for knights. The knight’s short jumps make it ideally suited to a fixed square in the centre, where it controls eight squares and is supported by friendly pawns. A knight on a central outpost is often worth more than a bishop on the same board, because the bishop has many squares of one color but the knight controls a critical square that cannot be neutralised.
Bishops can occupy outposts too, but with less effect. A bishop on a central square is mobile and dangerous, but its diagonals can be blocked or neutralised in ways a knight’s jumps cannot. The most lasting outposts in chess are knight outposts, and recognising them — and creating them — is one of the foundational positional skills.