Maintainability

Source: developer, sysadmin, user
Stimulus: add/delete/modify function or quality
Artifact: UI, platform, environment, external system
Environment: design, compile, build, runtime, server in maintenance
Response: make change, test it, deploy it
Measure: effort, implementation time, cost, risk

Maintainability is how making changes are easy and fast (and, by extension, cheap).

It's not necessarily flexibility, because a system can be inflexible by design but still need to be maintainable.

Scenario

Responsibility: which component handle which operation ? Keep module size small, limit responsibility
Cohesion: relevant distribution of responsibility (syntax and semantic)
Coupling: how dependents are components between them ?