Definition
A group method for producing many ideas within a time limit without judging them, then grouping and screening afterwards; it handles divergence and leaves judgement to the convergence step that follows.
In detail
Brainstorming never appears alone in the courseware; it always follows a structure — the skeleton of a fishbone diagram or one bar of a Pareto chart — otherwise ideas scatter across the whole problem space. The rules are no judging in the moment, quantity first, and building on other people's ideas; once divergence ends the group must switch moves and converge by grouping, voting or ranking by impact and difficulty, and the courseware typically narrows twenty or thirty items down to four to six. What it produces are hypotheses, not conclusions: root causes still have to be verified by observation and data.
Example in practice
Working from the Pareto bar holding eighty percent of the problem, the team produces 27 possible causes in ten minutes, groups them into six and votes four through, then checks each on the floor and confirms two.