Monochrome comes from the Greek μονόχρωμος (monochromos), meaning “one color”, which is a combination of μόνος (monos), meaning “alone” or “solitary”, and χρῶμα (chroma), meaning “color”. Monochromatic light is light of a single wavelength, though in practice it can refer to light of a narrow wavelength range. A monochromatic object or image is one whose range of colors consists of shades of a single color or hue; monochrome images in neutral colors are also known as grayscale or black-and-white.

In physics, the word "monochromatic" is used more generally to refer to electromagnetic radiation of a single wavelength. In the physical sense, no real source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration as a consequence of the Fourier transform's localization property (cf. spectral coherence).

Even sources such as lasers have some narrow range of wavelengths (known as the spectral linewidth) within which they operate. In practice, filtered light, diffraction grating separated light and laser light are all routinely referred to as monochromatic. Often light sources can be compared and one be labeled as “more monochromatic” (in a similar usage as monodispersity).

And a device which isolates light sources of a narrow bandwidth are called monochromators, even though the bandwidth is often explicitly specified, and thus a collection of wavelengths is understood. For an image, the term monochrome is usually taken to mean the same as black-and-white or, more likely, grayscale, but may also be used to refer to other combinations containing only tones of a single color, such as green-and-white or green-and-black. It may also refer to sepia displaying tones from light tan to dark brown or cyanotype (“blueprint”) images, and early photographic methods such as Ambrotype, Tintype and Daguerreotype, each of which may be used to produce a monochromatic image.

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