SAN Storage Area Network
A dedicated network giving servers access to storage at the block level. The server sees a LUN as if it were a local disk and puts its own filesystem on it.
That's the crucial difference from NAS, which serves finished files over NFS or SMB. Transport is traditionally Fibre Channel over a separate fabric with its own switches and HBA cards, with zoning and LUN masking deciding who sees what. Cheaper variants run iSCSI over ordinary Ethernet, and modern installations use NVMe over Fabrics for considerably lower latency. SAN is the foundation of traditional virtualization: shared block storage is what makes live migration of virtual machines possible. The trend still moves toward hyperconverged setups and toward object storage in the cloud, where the whole model looks different.