Order of Magnitude
A way of expressing how large a number is by which power of ten it is closest to — used for rough comparison, where a difference of one order of magnitude means roughly a tenfold difference.
Origin
The concept draws on the base-ten numeral system, which traveled from India through the Arabic world to Europe over centuries. The idea of classifying quantities by magnitude traces to ancient Greek astronomy — Hipparchus around 130 BCE and later Ptolemy ranked stars by brightness classes. German mathematician Paul Bachmann introduced formal O notation for describing orders of magnitude in his 1894 Die Analytische Zahlentheorie, and the term became standard in scientific usage by the early 20th century.
Everyday Use
When someone says a startup needs "on the order of" a million dollars, they mean roughly between $500K and $5M — the exact figure matters less than the scale. Orders of magnitude help you quickly compare things: Earth's population is four orders of magnitude larger than a small city's. It's the mental tool you use when asking "are we talking hundreds, thousands, or millions?"