Blast radius
Everything affected when one system goes down — including the systems that depend on it and did not know they did. What sets a window's real risk.
Also calledimpact radius · failure domain
The true reach of one outage: not the system you are taking down, but everything that depends on it.
The radius is almost always larger than the estimate, because dependencies accumulated over years and nobody wrote them down. A database host that looks peripheral turns out to hold a message queue three core systems read from.
Where the knowledge lives
In several places at once, which is the problem: in the dependency-monitoring tool, in two people's heads, and in an incident that happened once two years ago. Window planning that does not pull that knowledge into the moment the date is chosen discovers the radius in real time.