Think Tanks, by Stella Ladi. International Encyclopedia of Political Science*
"The term think tank has been used to describe both private and public organizations that provide advice to governments. A generally accepted definition would describe think tanks as organizations that are distinct from government and whose objective is to provide advice on a diverse range of policy issues through the use of specialized knowledge and the activation of networks."
Ladi, S. (2011). Think tanks. In B. Badie, D. Berg-Schlosser, & L. Morlino (Eds.), International encyclopedia of political science. (pp. 2608-2611). Thousand Oaks, CA: SAGE Publications, Inc. doi: http://dx.doi.org/10.4135/9781412959636.n604