Real Dictionary

pressure group
countable noun: A pressure group is an organized group of people who are trying to persuade a government or other authority to do something, for example to change a law.

pressure group in American English
any group that exerts pressure upon government officials and the public through lobbies, propaganda, etc. in order to affect legislation or policies
noun: an interest group that attempts to influence legislation through the use of lobbying techniques and propaganda

pressure group in British English
noun: a group of people who seek to exert pressure on legislators, public opinion, etc, in order to promote their own ideas or welfare