By atoms they might mean runtime-wide symbols that evaluate to themselves, kinda like a singleton that evaluates to itself. Useful for tagging data or states, i

By atoms they might mean runtime-wide symbols that evaluate to themselves, kinda like a singleton that evaluates to itself. Useful for tagging data or states, in BEAM-languages like Erlang or Elixir you commonly use the atoms :ok and :error to tag tuples to make it easy to switch or pattern match into appropriate receiving branches. You'll also come across them in Lisp-like languages and logic programming. Edit: Another, maybe better, illustration from the BEAM might be the 'booleans', they're implemented as the atoms :true and :false.