Astroturfing

Astroturfing is the practice of hiding the sponsors of a message or organization (e.g., political, advertising, religious, or public relations) to make it appear as though it originates from, and is supported by, grassroots participants. It is a practice intended to give the statements or organizations credibility by withholding information about the source's financial backers.

The implication behind the use of the term is that instead of a "true" or "natural" grassroots effort behind the activity in question, there is a "fake" or "artificial" appearance of support.