The percentage of times an ad was actually shown, out of the total number of times it was eligible to be shown, given its targeting and budget — a measure of how much of the available opportunity an advertiser is actually capturing.
Why it matters
A low impression share can indicate either a budget constraint (an ad is eligible to show more often than the budget allows) or a competitiveness constraint (a low bid or ad quality score losing auctions to competitors) — understanding which cause applies changes what action actually fixes it.
In a sentence
"Our impression share was capped by budget, not by bid competitiveness, so increasing daily spend was the right fix rather than raising our bids."
Related terms
- Often confused with: Reach (impression share measures the percentage of *available, eligible* impressions actually captured; reach simply measures the total number of unique people an ad was shown to, without reference to how much more opportunity existed beyond that).