What does the name mono mean?

It is actually a prefix meaning "one". A monorail is a train running on one rail. A monomaniac is a person obsessed with one thing. A monarch is a single person who is a ruler of a country. A monotone is a single tone, and a monotonous person or speech is one which has only one tone--very boring. A monopoly is a sector of the economy owned by one person or corporation, and the board game is all about becoming the sole owner of all the property on the board. A monograph is a specific study of a single specialized topic, usually by one researcher. The disease called mononucleosis is often called "mono" for short by those who find the whole name a bit of a mouthful.