Architecture Decision Log
also called ADR Log, Decision Register
The ordered, immutable collection of a system's decision records, read as a history rather than as a specification.
A single ADR answers "why is this one thing like this". The log answers a different and larger question: how did this system come to be the way it is? That is what a new architect needs and what no current-state document can provide.
What makes a log usable rather than an archive: sequential numbering so decisions can be cited, explicit supersession (a new record referencing the one it replaces, and the old one marked superseded but never deleted or edited), storage in the repository next to the code, and an index.
The most common failure is a log that stops. Ten records from the first six months and nothing after is worse than none, because it implies the early decisions are still current when several have been quietly reversed. If the log is not being added to, either decisions are not being made deliberately or the threshold for writing one is set too low and it has become a chore.