Order of magnitude
Definition
A rough size estimate to the nearest power of ten — answering 'about how big?' instead of 'exactly how big?' It is the art of the quick sanity check.
Technical definition
An estimate of a quantity's scale expressed as the nearest power of ten (10^n). Two values differ by an order of magnitude when their ratio is about ten; Fermi problems use chained order-of-magnitude estimates to approximate answers with minimal data.
Example
How many ping-pong balls fit in a school bus? A bus is about 10^1 m³, a ball about 10^-4 m³, so roughly 10^5 balls — a hundred thousand. The exact number hardly matters; knowing it is hundreds of thousands, not hundreds, is the point.
Order-of-magnitude is not sloppy guessing — it is disciplined estimation where each step is justified to within a factor of a few. And 'an order of magnitude larger' means about ten times larger, not just 'a lot larger' as everyday speech suggests.