Impressions

TL;DR

Impressions represent the number of times an ad or a page is displayed to users through third-party channels, such as Ads, Google Network, Social media ads, or in SERP (Search Engine Result Pages).

What is an impression and how is it tracked?

Impressions represent the number of times that an advertisement has been shown through Google AdWords, Google Network, and other forms of PPC advertising or the number of times a link is displayed on a SERP.

An impression is counted:

• Each time the website's advertisement appears on Google Search, Display, Maps or the Google Network
• Each time a person searches for a topic on the search engine and the website links appears as a result. Note that an impression is counted even when it hasn't been scrolled into view. 
• Every time a social media advertisement is shown to the target audience

What is the difference between Impressions and Reach?

Reach, as a metric, represents the total number of people who see your content or your ad. While Impressions are the number of times the content is displayed, no matter if it was shown to the same person multiple times or if it was clicked or not.