The Impression pixel is a peice of code that tracks the views of a creative on a website.  LinkTrust can place this code into creatives for affiliates by placing a Token in the creative.  When viewed through the Affiliate Center, the Toekn is replaced with the appropriate HTML pixel code which tracks impressions.

The Impression Token appears as [=ImpressionPixel=] in creatives within the Partner Center.

See also: Creative Tokens