A signal is a measurable change in voltage, current, or electromagnetic field strength used to convey information over a medium. In networking and telecom, signals can be analog continuous waves or digital discrete pulses. Engineers measure the integrity of a signal against the background noise to determine if the receiving equipment can accurately decode the transmitted message.
Heard on the job
The signal on that coax run is so degraded by the time it reaches the modem that it drops sync every five minutes.