A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at any given time it represents a real number within a continuous range of values.
A frequency-shift keying (FSK) signal is alternating between two waveforms and allows passband transmission. It is considered a means of digital data transmission.
Signal refers to both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In The Signal by William Powell Frith, a woman sends a signal by waving a white handkerchief.
A Finnish distant signal at the western approach to Muhos station is displaying Expect Stop.
A woman hailing a cab is sending a signal of availability to be picked up.
A flare is a common means to signal during dark or smoke-filled conditions.