A short control signal in telephony operations. It can be a single pulse, a brief interruption of a continuous tone, a change of bits or a change in polarity of the signal. For example, a momentary interruption (the wink) of a continuous, single-frequency tone is a signal that the receiving device is ready. The momentary interruption (the wink) of current to the light next to a telephone's hold button causes it to flicker as a signal to the user that the line is on hold.