On the morning of August 2nd, Google released a brand new help document that defines the metrics used on the Google Search Analytics report that webmasters can find within the Google Search Console.
This document aims at defining three core metrics used on the report
The document describes how these metrics charge based on the type of result. So, as an example, an impression for a basic blue link in the search results is counted when the page loads, but not when the user scrolls past the result. But an impression for a result in a carousel only is counted when the user scrolls past the result.
Google wrote the definitions in this document to act as a living document, so the definitions may end up changing. Google wrote, “The heuristics described here — such as the visibility requirement for an item in a carousel, or the position numbering—are subject to change.”
Make sure to check out the document over here.