You can picture a ladder of sizes where every step up multiplies by ten. Standing one step higher is being 'an order of magnitude' bigger; standing two steps higher is about a hundred times bigger. The ladder is what the phrase is really about — not the exact number you land on, but which step of the ladder you are standing on. That is why the phrase can be used with real numbers (ten times faster) and, just as easily, with no numbers at all (a different order of magnitude).
The word 'magnitude' came into this phrase from astronomy, where a star's magnitude was its class of brightness. Eighteenth-century use kept that idea of sorting things into classes by size, and the classes later became the steps of ten we use today.
「magnitude」這個字是從天文學來的：星星的 magnitude 就是它的亮度等級。十八世紀的用法保留了「按大小分等第」這個畫面，後來這個等第被借去講任何數量，一階固定成十倍。
Wiktionary, "order of magnitude"
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十倍一階的量級
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ten times bigger or smaller · noun
A step of ten times in size or amount. If one thing is an order of magnitude bigger or smaller than another, it is about ten times bigger or smaller, and two orders of magnitude means about a hundred times.
一個數量級（十倍）