According to Lehman and Ramil (chapter 1 of [MadhavjiEtAl2006]), the term evolution reflects "a process of progressive, for example beneficial, change in the attributes of the evolving entity or that of one or more of its constituent elements. What is accepted as progressive must be determined in each context.
It is also appropriate to apply the term evolution when long-term change trends are beneficial even though isolated or short sequences of changes may appear degenerative. For example, an entity or collection of entities may be said to be evolving if their value or fitness is increasing over time. Individually or collectively they are becoming more meaningful, more complete or more adapted to a changing environment."
The application of software maintenance activities and processes that generate a new operational software version with a changed customer-experienced functionality or properties from a prior operational version together with the associated quality assurance activities and processes, and with the management of the activities and processes[Chapin et al. 2001]