A cloud environment dedicated exclusively to a single organization. It can live on the company's own premises (on-premises) or be hosted by a provider, but resources are never shared with other customers.
In AZ-900 it is associated with greater control and higher cost. Common in regulated industries like banking or healthcare. In CLF-C02 it is contrasted with public cloud when discussing deployment models.
Private cloud is a concept that can show up on the exam. Check the full guide to see it in context.