Maximum harmonisation is a term used in European Union law.
If a piece of law (usually a directive, but also a regulation on occasion) is described as maximum harmonisation, that means that national law may not exceed the terms of the legislation. In practice, this prohibits gold-plating of European legislation when it is transposed into national law.
It may also result in the repeal or amendment of existing national law; for example the effect of the Unfair Commercial Practices Directive on the UK Trade Descriptions Act 1968
Traditionally it was fairly uncommon for European legislation to be drafted on this basis. However, deregulation has risen up the political agenda in EU member states, as have concerns that member states occasionally use the national implementation of European law as an opportunity to indulge in backdoor protectionism.
It is quite common for a directive or recommendation to consist of a mixture of maximum harmonisation and minimum harmonisation clauses.