Paywall
A paywall is a method of restricting access to content via a paid subscription. Beginning in the mid-2010s, newspapers started implementing paywalls on their websites as a way to increase revenue after years of decline in paid print readership and advertising revenue. In academics, research papers are often subject to a paywall and are available via academic libraries that subscribe.
Stonewall Attack
The Stonewall Attack is a chess opening; more specifically it is a variation of the Queen's Pawn Game. It is characterized by White (generally) playing their pawns to d4 and e3, playing Bd3, Nd2, and then playing pawns to c3 and then f4; although the moves are not always played in that order (see transposition). The Stonewall is a "system"; White heads for a very specific pawn formation, rather than trying to memorize long lines of different variations. If White puts up the Stonewall formation it is called "Stonewall Attack", regardless of how Black chooses to defend against it. When Black sets up a Stonewall formation, with pawns on c6, d5, e6 and f5, then it is a variation of the Dutch Defense. "MCO-15" gives the following as a main line: 1.d4 d5 2.e3 Nf6 3.Bd3 c5 4.c3 Nc6 5.f4. 
Since the Stonewall system is used against a variety of Black defenses, the "Encyclopaedia of Chess Openings" ("ECO") has trouble classifying it. Among the codes used are D00 (when Black has played ...d5), A45, and A03 (the code for Bird's Opening).