Said of software that is functional but easily broken by changes in
operating environment or configuration, or by any minor tweak to the
software itself. Also, any system that responds inappropriately and
disastrously to abnormal but expected external stimuli; e.g., a file system
that is usually totally scrambled by a power failure is said to be brittle.
This term is often used to describe the results of a research effort that
were never intended to be robust, but it can be applied to commercial
software, which (due to closed-source development) displays the quality far
more often than it ought to. Oppose robust.