Groupthink is 1) the practice of thinking or making decisions as a group in a way that discourages creativity or individual responsibility; 2) the process in which bad decisions are made by a group because its members do not want to express opinions, suggest new ideas, etc.
Reference Definition by Psysr.org: Groupthink, a term coined by social psychologist Irving Janis (1972), occurs when a group makes faulty decisions because group pressures lead to a deterioration of “mental efficiency, reality testing, and moral judgement”. Groups affected by groupthink ignore alternatives and tend to take irrational actions that dehumanize other groups. A group is especially vulnerable to groupthink when its members are similar in background, when the group is insulated from outside opinions, and when there are no clear rules for decision making.
Related Definitions in the Project: The Project Management
