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.
ImpactStory
Unpaywall is a browser extension, which find free versions of (paywalled) articles. In July 2018 it was reported to provide free access to 20 million articles, and about 47% of the articles that people search with Unpaywall. In June 2017 it was integrated into Web of Science and in July 2018 Elsevier announced plans the same month to integrate the service into the Scopus search engine.