“Impression share” is the number of impressions you received divided by the estimated number of impressions you were eligible to receive. It's The percentage of relevant searches an advertiser’s ad is able to appear in vs. the amount of times those ads actually appear. Impression Share can be lost due to budget and ad rank. Eligibility is based on your current ads' targeting settings, approval statuses, bids, and Quality Scores. This number is updated once a day.
It Used in Pay per click advertising, this metric refers to the percentage of times viewers have seen an advertiser’s ad, in relation to the total possible amounts that ad could have been seen. If an ad campaign’s impression share is 70%, then the ads showed 7 out of 10 possible times.