A document in a particular format, which captures an architecturally meaningful decision. The purpose of such documentation is to be able to share with new-joiners or others why a decision was made, the context at the time, and the general thinking process around it - in addition to the decision itself!
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Blog post: http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions
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Github repo with templates, details and more: https://github.com/joelparkerhenderson/architecture_decision_record