monochrome

having one color; a painting, design, photo or outfit that is only one color

monocle

an eyeglass for only one eye

monolingual

speaking or writing only one language

monolith

something made out of one single large block or piece of stone; one single stone monument

monologue

a long speech given by only one person

monophobia

an abnormal fear of being the only one there; fear of being alone

monopoly

control of a product or service by one company

monorail

a track for subway or train cars that have only one rail

monosyllabic

having only one syllable

monotone

a speech in which every word has one tone of voice