In electronics, a signal is an electric current or electromagnetic field used to convey data from one place to another. Data is superimposed on a carrier current or wave by means of a process called modulation . In recent years, digital modulation has been getting more common, while analog modulation methods have been used less and less. There are still plenty of analog signals around, however, and they will probably never become totally extinct. Except for DC signals such as telegraph and baseband, all signal carriers have a definable frequency or frequencies.
Signals also have a property called wavelength , which is inversely proportional to the frequency.
In telephony, a signal is special data that is used to set up or control communication. See signaling .