Communications of the ACM: You Don’t Know Jack About Software Maintenance:

Software maintenance is not like hardware maintenance, which is the return of the item to its original state. Software maintenance involves moving an item away from its original state. It encompasses all activities associated with the process of changing software. That includes everything associated with “bug fixes,” functional and performance enhancements, providing backward compatibility, updating its algorithm, covering up hardware errors, creating user-interface access methods, and other cosmetic changes.

In software, adding a six-lane automobile expressway to a railroad bridge is considered maintenance–and it would be particularly valuable if you could do it without stopping the train traffic.

Related: Slashdot thread