Devolution is the statutory granting of powers from the central government of a sovereign state to government at a subnational level, such as a regional, local, or state level. Devolution can be mainly financial, e.g. giving areas a budget which was formerly administered by central government. However, the power to make legislation relevant to the area may also be granted.
Devolution differs from federalism in that the devolved powers of the subnational authority may be temporary and ultimately reside in central government, thus the state remains, de jure unitary. Legislation creating devolved parliaments or assemblies can be repealed or amended by central government in the same way as any statute.
Federal systems, or federacies, differ in that state or provincial government is guaranteed in the constitution. Australia, Canada, India, Mexico and the United States have federal systems, and have constitutions (as do some of their constituent states or provinces). They also have territories, with less power and authority than a state or province.
Read more about Devolution: Australia, Canada, France, Spain, United Kingdom, Movements Calling For Devolution, List of Unitary States With Devolution, Other Meanings of The Term Devolution