Definition:

An interest group (also called an advocacy group, lobbying group, pressure group, or special interest) is a group, however loosely or tightly organized, that is determined to encourage or prevent changes in public policy without trying to be elected. The textbook used in class defines this as "An organization of people with shared ideas and attitudes who attempt to influence public policy."
definition taken from ~ -whitney.com/americancitizen/links/lobbies.htm