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 4,320,834 websites that are Google Analytics Event Tracking Customers. We know of 2,490,736 live websites using Google Analytics Event Tracking and an additional 1,830,098 sites that used Google Analytics Event Tracking historically and 1,681,668 websites in the United States.
Get a list of 4,320,834 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 2,490,736 currently live websites and an additional 1,257,527 domains that redirect to sites in this list. 1,830,098 sites that used this technology previously and 1,681,668 websites in the United States currently using Google Analytics Event Tracking.