The surprisingly large amount of work
that needs to be done as the result of any small but globally
visible program change. "The world" may mean the entirety of some
huge program, or may in theory refer to every program of a certain
class in the entire known universe. For instance, "Add one #define
to stdio.h, and you have to recompile the world." This means that
any minor change to the standard-I/O header file theoretically
mandates recompiling every C program in existence, even if only to
verify that the change didn't screw something else up. In practice,
you may not actually have to recompile the world, but the
implication is that some human cleverness is required to figure out
what parts can be safely left out.
