1 - a group of people that seeks to influence public policy on the basis of a particular common interest or concern.
1-Interest group, also called special interest group or pressure group, any association of individuals or organizations, usually formally organized, that, on the basis of one or more shared concerns, attempts to influence public policy in its favour.
2 - interest groups share a desire to affect government policy to benefit themselves or their causes. Their goal could be a policy that exclusively benefits group members or one segment of society (e.g., government subsidies for farmers) or a policy that advances a broader public purpose (e.g., improving air quality).