Something which is monochromatic has a single colour. In physics, the word is used more generally to refer to electromagnetic radiation of a single wavelength.
For an image, the term monochrome is essentially the same as black-and-white, but the monochrome may be preferred to indicate that combinations such as green-and-white, green-and-black, etc., are not excluded.
In computing, monochrome has two meanings: it can mean having only one colour which is either on or off, or also allowing shades of that colour, although the latter is more correctly know as greyscale. Thus it too has some ambiguity.
A monochrome computer display is capable of displaying only a single colour, often green, amber, red or white, and often also shades of that colour.
In the physical sense, no real source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration. Even sources such as lasers have some narrow range of wavelengths (known as the linewidth or bandwidth of the source) within which they operate.
The word monochromatic comes from the two Greek words mono (meaning "one"), and chroma (χρωμα, meaning "surface" or "the colour of the skin").
See also: Monochrome BBS.
