Impression Share is the percentage of eligible impressions an ad received compared with the total impressions it could have received. A common calculation is: Impressions received ÷ eligible impressions.
It matters because paid search can spend budget quickly. Knowing this term helps teams control intent, targeting, bidding, creative, conversion tracking, and lead quality instead of optimizing for surface-level clicks.
Example
Suppose a campaign receives 800 impressions out of an estimated 2,000 opportunities for which it was eligible. Its impression share is 40%. The manager can then investigate budget and rank limitations rather than interpreting the percentage as a share of all searches in the market.