Blast radius (code)
The blast radius of a change is all the code that could break because of it: everything that depends on the thing you changed, directly or through other code. The larger the blast radius, the more places a change can break.
The idea, explained
The term is borrowed from physics and incident response, where blast radius describes how far damage spreads from a single point. In software it answers a specific question before you touch anything: if I change this, what else has to change with it? For a function, that's its callers; for a module, its importers; for a database column, every query and serializer that reads it. A leaf utility that nothing imports has a small blast radius and is safe to refactor freely. A core type that 40 files depend on has a large one, and an innocent-looking edit there can cascade into failures several files away.
Blast radius is usually computed from the dependency graph. Direct dependents are the immediate importers or callers. Transitive dependents are everything reachable by following those edges further out. Many teams approximate it with a simple count of incoming references (fan-in), which is cheap to compute and good enough to flag the high-risk hubs. The more precise version walks the full reverse-dependency closure and is what static-analysis and impact-analysis tools (and good code review) try to surface.
Two things make raw dependency edges an incomplete picture. First, dynamic dispatch, reflection, dependency injection, and string-keyed lookups create coupling that import graphs miss, so a static blast radius can undercount. Second, files that aren't connected by an import edge can still need to change together, a test and the code it covers, a schema and its migration. Co-change history (files that repeatedly appear in the same commits) captures this hidden coupling and is a useful complement to the static graph.
For developers, blast radius is mostly a risk-sizing and scoping tool. It tells you whether a change is a one-line fix or a multi-file migration, where to focus tests, and which edits deserve extra review. It is an estimate, not a guarantee: the real blast radius is only fully known once you make the change and run the type checker, the linter, and the test suite.