In computer terms an enclosure is a case which contains something. Examples external HDD enclosures.
An enclosure is an area that is surrounded by a barrier. In computer land it often is a case which contains something.
One of the most commonly used case is an 'external HDD enclosure' which is a case which can contain a HDD and which often has plugs for power, eSATA, Firewire, SAS, Ethernet, USB or Thunderbolt.
An enclosure can also contain other things. E.g. external graphics cards, RAID arrays (optionally with a HW RAID card in the enclosure), etc. etc.