The number of positive-going zero crossings (and therefore half the total number of zero crossings) that the amplitude of a signal makes per unit time, or, in the case of a spatial signal (a picture), per unit of distance. The term is used mainly with regard to signals capable of taking only one positive and one negative value of amplitude, especially the simple case of +1 unit and −1 unit. Although the amplitude is usually discrete, the time (or space) coordinate may be regarded as discrete or continuous, depending on the application and the mathematical methods to be employed.
The term was originally applied to Walsh functions. In the case of Walsh functions, or any similar functions which are periodic but in which there are several zero crossings per period at unequal intervals, the number of zero crossings per period is called the normalized sequency.
Many concepts such as bandwidth, and processes such as filtering, which were originally defined in terms of frequency, can equally well be defined in terms of sequency. The sequency formulation is often handled more simply and more rapidly by discrete devices such as computers.
See also discrete and continuous systems.