Technically, monochrome means one hue. (dichrome = two, trichrome = three, etc.) Technically, this image is monochrome: http://e621.net/post/show/97457/2008-candra-cat-cats_-musical-eyes_closed-feline-m
Technically, greyscale is not monochrome, it's achrome - no hue at all.
On the other hand, people have been treating greyscale and monochrome as synonyms and probably will continue to do so forever no matter what.
How should I tag, e621? Or, should we declare monochrome as an alias for greyscale?
Updated by SnowWolf