A digital signal is a physical signal that is a representation of a sequence of discrete values (a quantified discrete-time signal),
for example of an arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal.
The term digital signal can refer to
-a continuous-time waveform signal used in any form of digital communication.
-a pulse train signal that switches between a discrete number of voltage levels or levels of light intensity,
also known as a a line coded signal, for example a signal found in digital electronics or in serial communications
using digital baseband transmission, or a pulse code modulation (PCM) representation of a digitized analog signal.
A signal that is generated by means of a digital modulation method (digital passband transmission), produced by a modem,
is in the first case considered as a digital signal, and in the second case as converted to an analog signal.
https://en.wikipedia.org/wiki/Digital_signal