Thevenin's Theorem

Any linear voltage network which may be viewed from two terminals can be replaced by a voltage-source equivalent circuit comprising a single voltage source E and a single series impedance Z. The voltage E is the open-circuit voltage between the two terminals and the impedance Z is the impedance of the network viewed from the terminals with all voltage sources replaced by their internal impedances.
Thevenin's Theorem says you can simplify any linear circuit, regardless of complexity, to an equivalent circuit with a single voltage source and series resistance connected to a load. As in the Superposition Theorem, it must be linear. In other words, passive components such as resistors, inductors and capacitors are okay. Non-linear components such as semiconductors, do not fall under this theorem.