miliardo

This is a genuine family, unlike most of the look-alike seams on these pages. Italian took mille, a thousand, and made milione, a great thousand; French built milliard on top of it for the next step up. Esperanto borrowed both, and the shared mili- is real history rather than coincidence. What is not real is the ending: -ard is a French suffix that came along for the ride, and there is no Esperanto affix -ard- to look up.

Now the part that will actually cost you money if you get it wrong. Esperanto counts on the long scale, the continental system: miliono is 10^6, miliardo is 10^9, and biliono is 10^12. Modern English uses the short scale, where a billion is 10^9. So English billion is Esperanto miliardo, and Esperanto biliono is a thousand times larger than the English word it resembles. This is the largest false friend in the number system and it is invisible in a sentence, because both readings parse.

Grammatically it behaves like miliono and unlike mil: it is a noun in all but name, so it takes -j and -n and it links to what it counts with da. Du miliardoj da homoj, never du miliardo homoj. See numbers for where that line falls across the whole system.