The dedicated channel, video bridge, or doc that the responders to a Sev-1 work in together for the duration of the incident.
A war room is the working space where the response to a Sev-1 (or Sev-2) incident lives, typically a dedicated Slack/Teams channel plus a continuously-open video bridge plus a shared incident doc. Every responder joins; the incident commander runs it; updates flow into the doc on a fixed cadence; customer comms get drafted there. The war room dissolves when the incident is declared resolved, and the doc becomes the primary input to the postmortem.
Incidents that span multiple teams require synchronous coordination: who's investigating what, what we've ruled out, what we're trying next. Asynchronous Slack threads break down at Sev-1 speed because nobody can read fast enough. A formal war room (with a designated facilitator, structured updates, and a single source of truth doc) is the discipline that turns a chaotic 30-person channel into an effective response.
See the part of the platform that handles war room in production.