Norton's theorem

Norton’s theorem tells us that any linear circuit with only voltage sources, current sources, and resistances can be replaced at terminals $A-B$ by an equivalent current source in parallel with some resistance $R_{th}$.

It’s heavily interlinked with Thevenin’s theorem — they are the same circuit but with a source transformation. The equivalent resistance is the same. A handy set of equations are:

$$i=-\frac{v_o}{R_{th}}+i_{sc}$$ $$R_{th}=\frac{v_{oc}}{i_{sc}}$$

It may be a good idea to just learn exclusively Thevenin’s theorem and transform the circuit when asked for a Norton equivalent network.