mono

The root mono comes from Greek monos, meaning "alone, single, only." It is one of the cleanest roots in English: across almost its entire family, the prefix really does mean exactly "one," so once you spot mono- you can usually trust it.

Start with power. monos + arkhein (to rule) gave monarch — literally "one who rules alone" — and monarchy, rule by a single sovereign. Move to the market and monos + polein (to sell) gives monopoly: a situation where only one party gets to sell. One ruler, one seller — the same idea of a single dominant figure.

The most surprising member hides its prefix. monk comes from Greek monakhos, "a solitary person" — early Christian ascetics who withdrew to live alone. From that solitary figure grew monastery (the place where they lived), monastic (their way of life) and monkish. The shaved-head holy man you picture is, etymologically, just "the man who lives by himself."

Then come the "one X" words, which read like a checklist. Speech alone → monologue (one person speaking, the opposite of dialogue) and monotone (one unchanging pitch). When one tone drags on, you get monotonous and monotony — boredom is literally "one-note-ness." One spouse → monogamy / monogamous (monos + gamos, marriage). One color → monochrome / monochromatic (monos + khrōma, color). One stone → monolithic (monos + lithos, stone): something so massive and uniform it seems carved from a single block. And the modern technical batch just snaps "one" onto a noun: monorail (one rail), monolingual (one language), monoxide (one oxygen atom), mononuclear (one nucleus), monolayer (one layer).

The whole family follows one rule: find the thing after mono-, and put "single / only one" in front of it.