open resistance against the orders of an established authority that seeks to gain concessions from an oppressive power

A rebellion is when people refuse to obey orders and fight against authority. Those who do this are "rebels". They may be citizens of a country who try to take over the government by force because they do not trust the current system. Some rebels in history were slaves who fought back against their masters or against slavery in general. Leader of slave rebellions include Spartacus and Nat Turner.

Painting of Pier Gerlofs Donia and Wijerd Jelckama fighting for the freedom of his people

An example would be the Zhou dynasty in China. It made the rulers of other Chinese states that it captured live with their own families in the its capital, Anyang. However, it failed to stop rebellion this way because groups of lords were brought together that could rebel. The Zhou dynasty declined because other lords who were ruling parts of China rebelled.

Rebellion is a usual method of secession.