Coexistence is a term used to describe non-integrated Hybrid Cloud delivery.  If an organisation uses (for instance) on-premises delivery for most IT services but makes use of Dynamics CRM Online as a SaaS (Public Cloud) service in a standalone, non-integrated way, this approach would be a textbook example of coexistence.

For usage to be considered Hybrid, some form of integration is necessary which is not present in a coexistent scenario.

At the time of writing, coexistence remains the most common consumption model for Cloud services.