A "Demijohn or Demi-john" is a glass carboy encased in a plastic basket for ease of carrying.
54 liter (14.3 USG)

The most common sizes of demi-johns are 6.5 gallons US (25 L) and 14 gallons US (54 L). They are often used for bulk maturing. The 54 L size is also used for double-batching (fermenting 2 or 3, 5-6 gallon kits of wine or beer).