Ruby language feature (authored by Shugo Maeda) that allows monkey patches to be activated only in a specific lexical scope via `using`. Introduced as experimental in Ruby 2.0, made non-experimental in 2.1, but adoption remains limited. Nick Sutterer dismisses refinements as effectively unused because they are hard to understand and don't solve the problem of customizing objects at runtime/compile time. Recommended use cases elsewhere include dependency reduction and coordinated modernization.