Definition: One visitor can view many ads. A raw impression is counted each time the ad loaded/displayed to the visitor. A unique impression is counted only for a single machine viewing a particular ad within a 24hr period. After 24hr of inactivity (not seeing this ad), this visitor is counted again as a new unique impression. The reports for unique impressions/IPs show the activeness of your visitors. Counting unique impressions depend on the information from cookie stored on visitors' browser. Without cookie, a raw impression is always counted as an unique impression.
Example: Visitor A viewed Ad1, Ad2, Ad3, Ad2. Visitor B viewed Ad1, Ad4. Ad1: 2 unique, 2 raw impressions, Ad2: 2 raw, 1 unique, the rest has 1 unique and 1 raw.
Relationship: Unique impression is always smaller than the raw number as one visitor usually sees more than one ad.
Similar terms: imp, exposure, view. Raw impression will be the default term and simply referred to as impression.
Raw/Unique Visitor "Definition: All visitors from any IP are counted as raw visitors. Relationship: This number is always higher than the number..." 36% relevant in ReportingStatistics Reports
Reporting 
