From Longman Dictionary of Contemporary Englishspecial interest groupˌspecial ˈinterest ˌgroup noun [countable] PPGa group of people who share the same political or business aims, and who try to influence the government to help them with those aims
Examples from the Corpus
special interest group• Recruiting the practices through a special interest group and through self selection is likely to have biased results.From Longman Business Dictionaryspecial interest groupˌspecial ˈinterest group abbreviation SIG a group of people who all share the same aims, especially one that tries to influence government policy on a particular issuespecial-interest groups, such as the American Insurance Association and the National Venture Capital Association → group 