Monocle
An eyeglass for one eye
Monotone
A speech in which every word has one tone of voice
Monosyllabic
Having only one syllable
Monorail
A track or subway or train cars that only has one rail
Monopoly
Control of a product or service by one company
Monophobia
An abnormal fear of being the only one there
Monologue
A long speech given by one person
Monolingual
Speaking or writing a one language
Monochrome
Having one color
Monolith
Something made out of one single large block of stone