greyscale

Images of this sort are composed exclusively of shades of neutral gray, varying from black at the weakest intensity to white at the strongest.
Grayscale images are distinct from black-and-white images, which in the context of computer imaging are images with only two colors, black and white (also called bilevel, binary images); grayscale images have many shades of gray in between. In most contexts other than digital imaging, however, the term "black and white" is used in place of "grayscale"; for example, photography in shades of gray is typically called "black-and-white photography". The term monochromatic in some digital imaging contexts is synonymous with grayscale (as it denotes absence of any concrete hue), and in some contexts synonymous with black-and-white.
Grayscale images are often the result of measuring the intensity of light at each pixel in a single band of the electromagnetic spectrum (e.g. infrared, visible light, ultraviolet, etc.), and in such cases they are monochromatic proper when only a given frequency is captured. But also they can be synthesized from a full color image; see the section about converting to grayscale.