class Module

m.anonymous? # => false
m.name # => "M"
M = m # m gets a name here as a side-effect
m.anonymous? # => true
m = Module.new # creates an anonymous module

via the +module+ or +class+ keyword or by an explicit assignment:
A module gets a name when it is first assigned to a constant. Either

M.anonymous? # => false
module M; end

Module.new.anonymous? # => true

+anonymous?+ method returns true if module does not have a name, false otherwise:

m.name # => nil
m = Module.new

M.name # => "M"
module M; end

A module may or may not have a name.

def anonymous?
  name.nil?
end