Is one zillion a number

No, "zillion" is not a number with any mathematical definition it's informal language used to express an extremely large, indefinite amount. Unlike million, billion, or trillion, which have precise numerical values, "zillion" serves purely as exaggeration or emphasis in casual speech.

People use "zillion" colloquially when they want to emphasize an uncountably large quantity without specifying an exact number, such as "I've got a zillion things to do" or "this would cost a zillion dollars." The term has no place in formal mathematics, science, finance, or any technical field. Similar informal terms like "gazillion," "bazillion," or "umpteen" function the same way—they're rhetorical devices, not mathematical constructs. For actual large numbers beyond trillion, mathematics provides systematic naming: quadrillion, quintillion, sextillion, and so on, each with precisely defined values. If precision matters in your context, avoid "zillion" and use properly defined number terms instead.