Maintenance - Altering programs after they have been in use for a while. Maintenance programming may be performed to add features, correct errors that were not discovered during testing, or update key variables (such as the inflation rate) that change over time.
NIST SP 800-79-1
NIST SP 800-82 Final Draft
Maintenance - Any act that either prevents the failure or malfunction of equipment or restores its operating capability. [The Automation, Systems, and Instrumentation Dictionary, 4th Edition, ISA, 2003.]