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 3,032,185 websites that are Google Analytics Event Tracking Customers. We know of 866,320 live websites using Google Analytics Event Tracking and an additional 2,165,865 sites that used Google Analytics Event Tracking historically and 338,241 websites in the United States.
Get a list of 3,032,185 websites using Google Analytics Event Tracking which includes location information, hosting data, contact details, 866,320 currently live websites and an additional 1,210,731 domains that redirect to sites in this list. 2,165,865 sites that used this technology previously and 338,241 websites in the United States currently using Google Analytics Event Tracking.