Impression share is the percentage of the impressions available for a search term that your ads actually received. A low share on a keyword that converts profitably means there is volume you are not buying, usually because your bid or your daily budget takes you out of the auction. Amazon exposes this for brand-registered sellers, including a separate figure for the top-of-search placement.
What it tells you that ACOS cannot
ACOS describes the efficiency of what you bought. Impression share describes the size of what you did not buy. A keyword at 15% ACOS and 12% impression share is not a success story — it is a keyword you are barely present on.
Profitable keyword plus low impression share is the highest-confidence case available for raising a bid or lifting a budget cap.
Reading it honestly
Low share can come from bid, from budget exhausting early in the day, or from relevance. Check budget first — a campaign that stops delivering at midday shows poor impression share no matter what you bid.
High impression share is not automatically good either. Owning nearly all impressions on a term that converts badly means you have cornered a market you did not want.
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