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 3,227,630 websites that are Google Analytics Event Tracking Customers. We know of 946,221 live websites using Google Analytics Event Tracking and an additional 2,281,409 sites that used Google Analytics Event Tracking historically and 373,182 websites in the United States.
Get a list of 3,227,630 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 946,221 currently live websites and an additional 1,210,515 domains that redirect to sites in this list. 2,281,409 sites that used this technology previously and 373,182 websites in the United States currently using Google Analytics Event Tracking.