"Stonewall" is the term for a type of Queen's Pawn opening pawn structure on
the Q/K/KB files of either or both players. For White the pawns
occupy the 3 dark squares Q4+K3+KB4 (d4,e3,f4), and for Black
they occupy the 3 light Q4+K3+KB4 (d5,e6,f5).
Typically Black sets up the formation to obstruct attacks, as
in the Dutch or
in the Semi-Slav defences, while White may
adopt it to secure the centre before attacking on the king's-side as
in a Queen's Pawn Game variation.