A dipole is an electronic component having two poles (two terminals). If we call I the current that enters via one terminal and exits via the other, and V the voltage between the two terminals, one can define the characteristic of the dipole as the function that relates I and V. This function, I = f(V), and its associated curve constitute a sort of  “identity card” for the component.

An electronic circuit is the connection, terminal to terminal, of two dipoles. The two components are carrying the same current and are experiencing the same voltage (potential difference). The bias point of a circuit, also called the quiet point (Q-point), is the pair of values (V_Q,I_Q) which simultaneously corresponds to the characteristic of the two dipoles. This point, if it exists, is found at the intersection of the two characteristic curves of the two dipoles.