A paywall is a means of limiting access to material, usually news, by requiring a purchase or a paid membership. Beginning in the mid-2010s, newspapers began adopting paywalls on their websites as a means of increasing income following years of declines in paid print reading and advertising revenue, which were partially caused by the widespread use of ad blockers on computers and mobile devices. In academia, research articles are often locked behind a paywall and only accessible via academic libraries that have subscribed to the journal.
Paid access to online material has also been used to increase the number of print subscribers; for example, some newspapers charge a cheaper price for access to online content combined with delivery of a Sunday print edition than they do for access to online content alone. It's a strategy used by newspaper websites such as those of The Boston Globe and The New York Times in order to enhance both online income and print readership (which in turn provides more ad revenue).