Mono- means one

monochrome (adj) - having one color; a painting, design, photo or outfit that is only one color or shades of one color

monocle (n) - an eyeglass for one eye

monolingual (adj) - speaking or writing only one language

monolith (n) - something made out of one single large block or piece of stone; one single stone monument

monologue (n) - a long speech given by one person

monophobia (n) - an abnormal fear of being the only one there; fear of being alone

monopoly (n) - control of a product or service by one company

monorail (n) - a track for subway or train cars that only has one rail

monosyllabic (adj) - having only one syllable

monotone (n) - a speech in which every word has one tone of voice