Organisms are the highest level component that's reusable. They often wrap one or more molecules and atoms for the purpose of bridging them and your data layer. E.g. with flux organisms are the controller components which create actions and subscribe to stores.
They are very coupled to your data layer and are only reusable within the target domain. An example of an organism would be a wrapper around a comment box, or a component that takes a user id as a prop and renders the user's name. They can be large like a login form or as small as rendering a single word.