Mono- is a prefix that means “one” or “single.” When you see it at the start of a word, it tells you that something involves just one of whatever comes next.

A monopoly happens when one company controls an entire market. A monorail is a train that runs on a single rail instead of two. A monologue is when one person speaks alone for an extended time, like when an actor delivers a speech on stage. If something is monochrome, it uses only one color, like an old black-and-white photograph.

You might hear someone say they caught mono, which is short for mononucleosis, an illness that can make you feel very tired and sick. Or you might hear about someone who speaks in a monotone, meaning their voice stays at one flat pitch without rising or falling.

A related prefix is poly-, meaning “many.” So while a monolingual person speaks one language, a polylingual person speaks many.