What is brainstorming and when is it most/least effective?

A timed group session (5–10 people) where a moderator collects ideas without judging them, and participants riff on each other's ideas — most effective with diverse groups, least effective when dominance or shyness skews participation.

Brainstorming: Ideas are collected within a certain time frame, usually in groups of 5 to 10 people. The ideas are documented by a moderator without discussing, judging, or commenting on them at first. Participants use ideas of other participants to develop new original ideas or to modify existing ideas. After that, the collected ideas are subjected to a thorough analysis.

Most effective when:

• Large number of people of different stakeholder groups are involved
• Need to collect large number of ideas in short time
• Multiple people can expand on ideas collaboratively
• Unbiased collection allows new solutions to pop up

Less effective when:

• Group dynamics are muddied
• Participants have very varied levels of dominance
• People are not comfortable speaking up

Go deeper:

• Brainstorming (Wikipedia) — the group idea-generation technique, its "defer judgment" rule, and its known productivity limits.