“In the US context, the term “think tank” is generally used to refer to an organization that (1) has the provision of research and policy advice as its primary mission; (2) is organizationally—although sometimes not financially—independent of government and universities; and (3) is operated on a not-for-profit basis. But the boundaries between think tanks, advocacy organizations, and nongovernmental organizations (NGOs) that have the dissemination of public information as part of their mandate is often ambiguous in practice. Because of these uncertain boundaries, even the most basic question about think tanks in the United States—how many there are—cannot be answered with certainty. Following a strict version of the definition based on the three criteria above, there are more than 300 think tanks in the United States. James McGann, using a much broader definition, cites a figure of 1,816 US think tanks in the 2010 edition of his global think tank guide.”

Think Tanks - The Oxford Companion to American Politics