Runbook
A documented, step-by-step procedure for executing a specific operational task, most critically failover and recovery, so it doesn't depend on one person's memory under pressure.
Definition
A Runbook is a written, step-by-step procedure for carrying out a specific task, most importantly failover and disaster recovery, precisely enough that someone unfamiliar with the system could follow it during an incident. The value of a runbook is proportional to how recently it's been tested; an untested runbook written two years ago against an environment that's since changed is close to worthless when it's actually needed.
In Practice
A disaster recovery runbook should specify exact commands, exact accounts, and exact people to notify, not general guidance like 'restore from backup,' which leaves too much to be worked out during an actual outage.