Acronym for Storage Area Network.
A SAN lets you network disk devices to give you remote block level access to files; it effectively lets you access a remote disk as if it was a local disk in the computer you are using. Compare with NAS, where you remotely access a NetworkFileSystem. With a SAN, you request "block 6000 from disk drive 4"; with a NAS, you request "/tmp/example".
SANs often use the SCSI protocol to access the data.
See also:
WikiPedia:SAN