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 2,935,871 websites that are Google Analytics Event Tracking Customers. We know of 942,874 live websites using Google Analytics Event Tracking and an additional 1,992,997 sites that used Google Analytics Event Tracking historically and 385,993 websites in the United States.
Get a list of 2,935,871 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 942,874 currently live websites and an additional 1,294,698 domains that redirect to sites in this list. 1,992,997 sites that used this technology previously and 385,993 websites in the United States currently using Google Analytics Event Tracking.