URL hijacking is the practice by which a rogue affiliate creates a PPC search ad and display a destination URL (called display URL) as if the ad has been created by the affiliate merchant. More often, the fraudster affiliate copies an actual ad originally created by the merchant.
The hijacked ad routes visitors through affiliate links before landing on the merchant website. These ads are hard to detect because they look like the merchant ads.
URL hijacking is a form of trademark poaching.
An example of URL Hijacks Report by Search Monitor: