What is "Status: degraded" ?

Hi, all

I want to understand "Status: degraded".

But I can not see an explanation on CLI manual.

[FYI]

It state can see "esxcli storage core device list" command.

naa.60060480000290105757533030363837:
   Display Name: EMC Fibre Channel Disk (naa.60060480000290105757533030363837)
   Has Settable Display Name: true
   Size: 2
   Device Type: Direct-Access
   Multipath Plugin: NMP
   Devfs Path: /vmfs/devices/disks/naa.60060480000290105757533030363837
   Vendor: EMC    
   Model: SYMMETRIX      
   Revision: 5773
   SCSI Level: 4
   Is Pseudo: true
   Status: degraded <-----Here!!!!!
   Is RDM Capable: true
   Is Local: false
   Is Removable: false
   Is SSD: false
   Is Offline: false
   Is Perennially Reserved: false
   Thin Provisioning Status: unknown
   Attached Filters:
   VAAI Status: unknown
   Other UIDs: vml.0200fa00006006048000029010575753303036383753594d4d4554

Thanks