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 3,164,971 websites that are Google Analytics Event Tracking Customers. We know of 1,151,931 live websites using Google Analytics Event Tracking and an additional 2,013,040 sites that used Google Analytics Event Tracking historically and 588,849 websites in the United States.
Get a list of 3,164,971 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 1,151,931 currently live websites and an additional 1,289,001 domains that redirect to sites in this list. 2,013,040 sites that used this technology previously and 588,849 websites in the United States currently using Google Analytics Event Tracking.