Events are user interactions with content that can be tracked independently from a web page or a screen load.
https://developers.google.com/analytics/devguides/collection/analyticsjs/events
Analytics and Tracking
Get access to data on 2,943,069 websites that are Google Analytics Event Tracking Customers. We know of 869,884 live websites using Google Analytics Event Tracking and an additional 2,073,185 sites that used Google Analytics Event Tracking historically and 356,830 websites in the United States.
Get a list of 2,943,069 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 869,884 currently live websites and an additional 1,381,722 domains that redirect to sites in this list. 2,073,185 sites that used this technology previously and 356,830 websites in the United States currently using Google Analytics Event Tracking.