Definition of Affinity Diagram:
A tool used to organize and present large amounts of data (ideas, issues, solutions, problems) into logical categories based on user perceived relationships and conceptual frameworking.
Often used in form of sticky notes sent up to front of room in brainstorming exercises, then grouped by facilitator and workers. Final diagram shows relationship between the issue and the category. Then categories are ranked, and duplicate issues are combined to make a simpler overview.