a function of time, which means that it can be a varying entity with several variables, which must include time. As an example, a 2D image can be represented by a function g(x,y,t), where x and y are the co-ordinates on the image's plane, and t is the instant of observation. The function's value is the amplitude of the signal. This lesson only considers signals varying with time - one-dimensional signals. Theoretically, the signals can be classified as continuous or discrete.