Concept from Eric Evans' Domain-Driven Design book, cited as the closest DDD analogue to the Rails 'service object'. Key properties: stateless (take arguments, do work, return); named for an activity rather than an entity; represent a significant business domain process or operation; distinct from Technical Services; typically have side effects (or at least are expected to have them unless guarded by a condition). Ivan combines these rules with Fowler's Service Layer properties and Robert Martin's notes to build a 'service ruler' for evaluating real-world Rails service objects — and finds none fully comply.