An impression is defined as the number of times your ad is displayed on either the SERP or Google display network site.
Definition Impressions are a metric which detail how many times an ad was shown within the SERP or Google display network. They tell you how many people saw your ad. Impressions can increase and decrease depending on the competition the keyword you're bidding on faces. The amount of impressions you garner depends on your impression share which is metric that gauges how much percent of the total impressions generated for a specific keyword that you are bidding on. Impressions are directly tied to click through rate (CTR) which is a measure of how many clicks you receive per impressions shown.
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