Manifest file shipped with every Ruby gem. Describes package identity (name, version, author, summary, description), metadata URLs used by RubyGems and other indexers (homepage, bug tracker), the list of files to include in the packaged gem (important for excluding dotfiles, node_modules, etc.), and runtime/development dependencies declared via `spec.add_dependency` / `spec.add_development_dependency`. Adrian frames it as 'the package.json of Ruby'.