Ternary signal

In telecommunication, a ternary signal is a signal that can assume, at any given instant, one of three significant conditions, such as power level, phase position, pulse duration, or frequency.

Note: Examples of ternary signals are (a) a pulse that can have a positive, zero, or negative voltage value at any given instant, (b) a sine wave that can assume phases of 0°, 120°, or 240° relative to a clock pulse, and (c) a carrier wave that can assume any one of three different frequencies depending on three different modulation signal significant conditions.

Source: from Federal Standard 1037C