Other forms: shills; shilled
A shill is someone who pretends to be a customer, but is really an accomplice. If you post a positive review online of your best friend's terrible taxi service, you're a shill.
In journalism, asking a politician extremely easy questions instead of the difficult ones they don't like answering is one way to be labeled a shill. And if you see someone win a hundred dollars from a sidewalk magician, they're almost definitely a shill, trying to entice others to gamble on the trick. This word was originally carnival huckster lingo, shortened from shillaber, of unknown origin.