Google has posted a help document named What are impressions, position, and clicks? This document is one of the most comprehensive and authoritative documents on how Google defines the metrics shown in the Google Search Console’s Search Analytics report.
It goes through the definition of an impression and in this case it is mostly when a search result screen loads your result, most of the time, the user doesn’t have to scroll past the result but in some cases they do for it to be counted as an impression.
It also goes through how Google defines position, including top position and average position. It also defines what constitutes a click. It goes through which URL Google assigns these metrics to. It then breaks it down by type, such as the carousel, AMP results, featured snippets, rich results, app install links and image results.
This document is incredibly comprehensive, I suggest all webmasters and SEOs read it a few times.
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Author: firstname.lastname@example.org (Barry Schwartz)