Digital signal

A binary signal, also known as a logic signal, is a digital signal with two distinguishable levels

The term digital signal has related definitions in different contexts.

In signal processing, a digital signal is an abstraction that is discrete in time and amplitude, meaning it only exists at certain time instants.
A logic signal waveform: (1) low level, (2) high level, (3) rising edge, and (4) falling edge.

Although in a highly simplified and idealized model of a digital circuit, we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finite transition time the output may not properly reflect the input, and will not correspond to either a logically high or low voltage.

To create a digital signal, an analog signal must be modulated with a control signal to produce it. The simplest modulation, a type of unipolar encoding, is simply to switch on and off a DC signal so that high voltages represent a '1' and low voltages are '0'.

Asynchronous logic also exists, which uses no single clock, and generally operates more quickly, and may use less power, but is significantly harder to design.