Signal

In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time‐varying or spatial‐varying quantity. This variable(quantity) changes in time.
Eg :-

•  Speech or audio signal: A sound amplitude that varies in time
•  Temperature readings at different hours of a day

Signals can be classified by continues‐time signal and discrete‐time signal :

• A discrete signal or discrete‐time signal is a time series, perhaps a signal that  has been sampled from a continuous‐time signal
• A digital signal is a discrete‐time signal that takes on only a discrete set of values