From Science Europe Data Glossary
Definition: software sustainability is the maintenance and preservation of software, aiming to keep it usable for the future.
Explanation: research data depends on software to be read and used. Because of technological changes (for instance in operating systems) storing and preserving software is in itself no guarantee for it’s continued usability. Maintenance of software implies that it continues to be updated, improved and supported in the future.
Reference: C. Venters et al., "Software Sustainability, The Modern Tower of Babel: Position Paper". RE4SuSy 2014: Third International Workshop on Requirements Engineering for Sustainable Systems, Karlskrona, Sweden, August 26, 2014, 6 pp. http://ceur-ws.org/Vol-1216/paper2.pdf