Vinny Lingham informs about a new metric which he discovered on Google AdWords console called Impression Share. He says, “Impression share is a metric available at the campaign and account level for search.”
Campaign: If an advertiser has more than one campaign competing in the same target market, the resulting impression share for any one campaign won’t be penalized if it loses an impression opportunity to another campaign from within the same account.
Account: Reports at this level will not penalize your impression share from the competition across all your campaigns.
For a detailed explanation checkout the post.