We are all immersed in a sea of signals. All of us from the smallest living unit, a cell, to the most complex living organism (humans), receive signals all the time and continue to process them. Survival of any living organism depends upon its ability to process the signals appropriately.

What is a Signal?

Anything which carries information is a signal. e.g. human voice, chirping of birds, smoke signals, gestures (sign language), fragrances of the flowers.

Many of our body functions are regulated by chemical signals, blind people use sense of touch. Bees communicate by their dancing pattern.

Modern high speed signals are: voltage changer in a telephone wire, the electromagnetic field emanating from a transmitting antenna,variation of light intensity in an optical fiber.

Thus we see that there is an almost endless variety of signals and a large number of ways in which signals are carried from on place to another place.

Signals: The Mathematical Way

A signal is a real (or complex) valued function of one or more real variable(s).When the function depends on a single variable, the signal is said to be one-dimensional and when the function depends on two or more variables, the signal is said to be multidimensional.

Examples of a one dimensional signal:  A speech signal, daily maximum temperature, annual rainfall at a place
An example of a two dimensional signal: An image is a two dimensional signal, vertical and horizontal coordinates representing the two dimensions.
Four Dimensions: Our physical world is four dimensional(three spatial and one temporal).