Decentralization is the practice of distributing authority among multiple smaller groups rather than a single large one. For instance, when the government is decentralized, it is distributed across the states rather than being concentrated at the federal level. Decentralization in business refers to a framework that distributes authority across several smaller organizations or places as opposed to a single, central organization.
In the world of cryptocurrency, decentralization is the basis for operations, as there is no central unit that controls operations or lets them happen or not in the first place. Bitcoin is decentralized as there is no central entity that prints or creates money, but rather the money is mined by a crowd of individuals.