Thick, Thin provisioning, and Over-allocation are mechanisms that apply to SAN volumes (iSCSI and Fibre Channel).
Thick provisioning (traditional allocation)
When creating (provisioning) a volume, the declared size of that volume is fully allocated at the moment of creating and cannot be used anymore by other volumes or datasets.
When provisioning a resource the actual declared size is not immediately allocated and rendered unavailable. Only the actual data put into that volume reduces the amount of available space.
In addition to thin provisioning it is possible to declare a volume with size larger than the pool on which it’s located, e.g. 100TB volume on 10TB pool.