A Logical Unit Number (LUN) is a number assigned to a logical disk unit formed at a storage array. It is a unit of storage that can be uniquely identified regardless of the storage protocol (FC/iSCSI/FCoE). The logical disk unit is also referred to as a LUN device. A LUN device is also assigned a unique NAA ID (Network Address Authority Identifier). A storage array is nothing but an array of hard disks either of the same type or of different types depending on the type, model, and the planned configuration of storage arrays. The disks are grouped into RAID groups to provide for performance and recoverability service level agreements. These RAID groups can cater storage space to one or more LUN devices:

In the diagram above, you will notice that the single RAID-10 group is catering storage space to a single LUN-A, whereas the RAID-5 group is catering to three LUNs - LUN-B, LUN-C, and LUN-D. An ESXi host can discover up to 256 LUN devices.