How do you define the thevenin methode?

Answer 1

Thevenin’s Theorem states that any electrical circuit or part of a circuit with voltage and current sources and resistors can be replaced by an equivalent ideal voltage source called Thevenin Source and a resistor in series with it called Thevenin Resistor.  It is also valid for AC circuits, where the voltage and current sources are replaced by AC sources and resistors are replaced by impedances.
 
It is widely used for circuit simplification when studying or calculating the circuit components, or for the calculation of circuit currents, transfer functions, input and output resistances etc.  For Thevenin’s Theorem application you can find a very good article here:

http://masteringelectronicsdesign.com/how-to-apply-thevenins-theorem-part-1/