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 3,333,431 websites that are Google Analytics Event Tracking Customers. We know of 993,749 live websites using Google Analytics Event Tracking and an additional 2,339,682 sites that used Google Analytics Event Tracking historically and 407,806 websites in the United States.
Get a list of 3,333,431 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 993,749 currently live websites and an additional 1,236,256 domains that redirect to sites in this list. 2,339,682 sites that used this technology previously and 407,806 websites in the United States currently using Google Analytics Event Tracking.