Amazon PPC glossary
Top-of-search impression share
The share of top-of-search impressions a campaign is capturing, reported separately from overall impression share because that placement carries most of the visibility and most of the cost.
Where it matters most: Book 5, The Advanced PPC Clarity Guide, in the chapter “Scaling or Protecting a Winning Campaign.”
Go deeper
The glossary gives you the one-line answer. The Advanced PPC Clarity Guide explains why it matters and what to do about it.