Mono is a prefix meaning “one” or “single” that comes from ancient Greek. You'll find it at the start of many English words where it signals that something involves just one thing rather than many.

A monorail is a train that runs on a single rail instead of two. A monotone voice stays on one note without varying in pitch, which can make a speaker sound pretty boring. When someone has a monopoly on something, they're the only one selling it, which means no competition. A monologue is when one person speaks alone for an extended time, unlike a dialogue where two people talk back and forth.

The prefix works the same way in science: carbon monoxide has one oxygen atom, while carbon dioxide has two. An image is monochrome if it uses just one color.

You might hear people say they have mono as shorthand for mononucleosis, an illness that makes you very tired.

Understanding mono helps you decode lots of words: if you see it at the beginning, you know you're dealing with something singular, solitary, or unified rather than multiple or divided.