A part in Hanami View is a Ruby class used to decorate values that the view exposes to a template. In Wilgosz's registration example, a RegistrationFormPart wraps a form hash and defines an `errors(name)` method that always returns an array (never nil) so templates don't need `to_a` transformations. Parts can also host methods like `username_input(form_builder)` that hide scope creation and partial rendering entirely, letting the top-level template read as a list of decorator calls.