What is the definition of interest groups?

An interest group, otherwise known as a "special-interest group," is defined by Encyclopedia Brittanica as "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 favor."
Interest groups could be economic or related to labor interest, such as the AFL-CIO. They could be cause groups, such as those who favor abortion rights (e.g., Planned Parenthood) or those who oppose them (e.g., Susan B. Anthony List). They could be groups that push for environmental protections (e.g., Sierra Club, National Resources Defense Council). They could also be groups that work toward social justice (e.g., NAACP, Southern Poverty Law Center) or for civil liberties (e.g., American Civil Liberties Union).
Interest groups attempt to influence legislation through lobbying—"that is, by attempting to bring pressure to bear on policy makers to gain policy outcomes in their favour." One of the ways in which they use their influence is by contributing to the political campaigns of politicians who are likely to share their views.
https://www.britannica.com/topic/interest-group

https://www.britannica.com/topic/interest-group/Types-of-interests-and-interest-groups