Brainstorming is a technique that has gone from the boardroom into the real world and is used by employees of nearly every type of company imaginable, and is also used by other types of groups of individuals when there is a desire to make a collective decision. More specifically, brainstorming is a technique in which multiple individuals (the number is irrelevant, it may be a large group, or it may be a group as small as two) gather to discuss a specific agenda item with the goal of coming up with a plan of action. During this process, all of the individuals are encouraged to suggest ideas which then may be considered and discussed by the group as a whole. In some cases all individual suggestions are recorded, and in others, only the ones that the team decides to go with are kept track of. Brainstorming is also a team building activity in that, typically, brainstorming sessions are non-judgmental, and participants are encouraged to offer any reasonable (or in some cases even unreasonable) idea for discussion without fear of reproach.’
This term is defined in the 3rd and the 4th edition of the PMBOK.