Definition:
Affinity Diagram is a powerful tool to organise information. A tool that gathers ideas, opinions, issues and organised them into groups or themes based on their natural relationships.
The term affinity diagram was devised by Jiro Kawakita in 1960 and also referred as KJ Method. During brainstroming session, this tool allows large number of ideas generated and sorted to make some sense for review and analysis to bring insights. The process to develop this is to record each ideas on cards or notes, cluster ideas that seems to be related and label them as it make sense which can be further use for easier management and analysis.
Further Reading:
https://en.wikipedia.org/wiki/Affinity_diagram
https://www.interaction-design.org/literature/article/affinity-diagrams-learn-how-to-cluster-and-bundle-ideas-and-facts