The logical sense, true or false, of a given binary signal. A binary signal is a digital signal that has only two valid values. In physical terms the logical sense of a binary signal is determined by the voltage level or current value of the signal, and this in turn is determined by the device technology. In TTL circuits, for example, a true state is represented by a logic 1, approximately equal to +5 volts on a signal line; logic 0 is approximately 0 volts. Voltage levels between 0 and +5 volts are considered undefined.

Since only two logic states, logic 1 and logic 0, are possible, the techniques of Boolean algebra may be used to analyse digital circuits involving binary signals. The term positive logic is applied to circuits where logic 1 is assigned to the higher voltage level; in negative logic circuits a logic 1 is indicated by the lower voltage level. See also multivalued logic.