A more thorough explanation:

The deviation doctrine is a legal principle that allows for variation from a term of a will or trust, or from the scope of a principal's permission for an agent's activity, in order to avoid defeating the document's purpose. In maritime law, the deviation doctrine refers to the rule that a carrier 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.

For example, if a person creates a trust that specifies that the funds are to be used for the education of their grandchildren, but one of the grandchildren decides not to pursue higher education, the trustee may be able to deviate from the terms of the trust in order to use the funds for the benefit of the other grandchildren.

In another example, if a principal gives an agent permission to sell a car for $10,000, but the agent is only able to find a buyer willing to pay $9,500, the deviation doctrine may allow the agent to go ahead with the sale in order to fulfill the principal's intent of selling the car.

In maritime law, if a carrier deviates from the terms of the bill of lading in a reasonable manner, such as to avoid a storm or other danger, they may still be able to claim the limitations and exemptions under the Carriage of Goods by Sea Act. However, if the deviation is unreasonable, such as taking a longer route for no good reason, they may lose those benefits.