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.
Invisible wall
An invisible wall is a boundary in a video game that limits where a player character can go in a certain area, but does not appear as a physical obstacle. The term can also refer to an obstacle that in reality could easily be bypassed, such as a mid-sized rock or short fence, which does not allow the character to jump over it within the context of the game. In 2D games, the edge of the screen itself can form an invisible wall, since a player character may be prevented from traveling off the edge of the screen.