An Architectural Decision is a justified design choice that addresses a functional or non-functional requirement that is architecturally significant. An Architecturally Significant Requirement is a requirement that has a measurable effect on the architecture and quality of a software and/or hardware system. An Architectural Decision Record (ADR) captures a single AD and its rationale; Put it simply, ADR can help you understand the reasons for a chosen architectural decision, along with its trade-offs and consequences. The collection of ADRs created and maintained in a project constitute its decision log. All these are within the topic of Architectural Knowledge Management (AKM), but ADR usage can be extended to design and other decisions (“any decision record”).
Templates
Some markdown ADR templates can be found here.