Questions that may not necessarily have a clear-cut right or wrong answer and are often subjective. If your question isn't a bug report, feature request, or request for assistance, or question with a concrete answer, it's probably a discussion.
This tag is used for questions that ask users to — what else? — discuss an idea, concept or problem. Common use cases include hashing out how the community should handle a certain action and collaborating on how to solve a perceived issue within the community. A discussion question may not necessarily have a clear-cut right or wrong answer and is often subjective.
discussion is one of the four possible mandatory tags. The other mandatory tag options are bug, feature-request and support.
Source (excerpt): https://meta.stackexchange.com/revisions/63164/12
Source (wiki): https://meta.stackexchange.com/revisions/62883/14
