To convert an analogue signal, such as speech, sound or light, into a numeric form which can be processed by a computer. For example, if you digitise speech you use an analogue-to-digital converter to convert the sound into a series of numbers that can be stored on disk and then played back. If you digitise an image, you use a camera to convert the reflected light into an array of dots, each one a number that represents the brightness at that point.
