A narrowboat or narrow boat is a boat

Boat

A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...

 of a distinctive design, made to fit the narrow canals of Great Britain

History of the British canal system

The British canal system of water transport played a vital role in the United Kingdom's Industrial Revolution at a time when roads were only just emerging from the medieval mud and long trains of pack horses were the only means of "mass" transit by road of raw materials and finished products The...

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In the context of British Inland Waterways, "narrow boat" refers to the original working boats built in the 18th, 19th, and 20th centuries for carrying goods on the narrow canals (where locks

Lock (water transport)

A lock is a device for raising and lowering boats between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water level can be varied; whereas in a caisson lock, a boat lift, or on a canal inclined plane, it is...

 and bridge holes would have a minimum width of 7 feet (2.1 m)). The term is extended to modern "narrowboats" used for recreation and occasionally as homes, whose design is an interpretation of the old boats for modern purposes and modern materials.
Purists tend to use the term with a space (narrow boat) when referring to an original boat or a replica, and to omit the space when referring to a modern boat used for leisure or as a residence - but this is not a hard and fast rule.