DAEDRIC FUN TIP:
Mortals cannot be trusted to make decisions on their own. Peruse other images to make sure you are tagging properly.

monochrome

Any image composed of one colour. Whether it's black and white, blue and white, red and white or anything else and white, as long as it's lacking color variety, it's monochrome.

Note:
Images that fit the above description but have color elsewhere (such as the example below) are still considered monochrome as we're only really interested in the subject of the image. Not the unrelated parts that may have also been photographed.