In the British Isles at least, the definition of a packhorse bridge is somewhat nebulous.
Ernest Hinchliffe discusses the difficulty of defining a true packhorse bridge in A Guide to the Packhorse Bridges of England. He claims that "before the eighteenth century bridges were described as 'horse bridges' or 'cart bridges'" and his strict definition excludes the latter. He suggests that a classic packhorse bridge should generally be:
- less than 6 feet (1.8 m) in width;
- built before 1800;
- on a recognised packhorse route