Deviation doctrine refers to a principle allowing variation from a term of a will or trust to avoid defeating the document’s purpose. It can also be a principle allowing an agent's activity to vary slightly from the scope of the principal's permission.
Doctrine of deviation is a long-standing feature of English maritime law. Deviation, in this context, means the voluntary and unjustified departure of a ship from her agreed route. In the law of carriage of goods by sea, deviation is treated as a breach of contract which has special consequences. According to this rule a loses the benefit of its limitations and exemptions under the Carriage of Goods by Sea Act if a deviation from the terms of the bill of lading is unreasonable, but does not if it is reasonable.