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 commercially developed software, which displays the quality far more often than it ought to.
Opposite of robust.