Architecture Decision Record
also called ADR
A short, immutable document capturing one architectural decision, its context, the alternatives, and its consequences.
An ADR answers the question that costs teams the most time: why is it like this? It is typically one page — context, the decision, the options considered, and the consequences accepted — written at the moment the decision is made, numbered, and never edited afterwards. Superseding a decision means writing a new ADR that references the old one.
The immutability is the point. A living design document tells you what is true now; a sequence of ADRs tells you what was believed and why at each point, which is what you need when the decision turns out to be wrong.
The most valuable section is "consequences", and specifically the negative ones. An ADR with no downsides listed is a decision that was not actually made.