Which logical entity in ACI allows for logical separation and reuse of IP address space...

In ACI (Application Centric Infrastructure), the logical entity that allows for logical separation and reuse of IP address space within an ACI fabric is the Tenant. A Tenant represents a unit of isolation from a policy perspective and allows for the segmentation of the network into multiple virtual environments, each with its own policies and IP address spaces.

References:

Cisco ACI documentation.

Technical resources on ACI architecture and logical entities.