order of magnitude
An order of magnitude is the size of a number rounded to the nearest power of ten, its ballpark. Instead of asking exactly how big something is, you ask which power of ten it sits near: tens, thousands, millions? Two quantities are 'of the same order of magnitude' if one is within about a factor of ten of the other.
Formally, the order of magnitude of a number is the exponent when you write it in scientific notation and round the coefficient: 4,000 is 4 times 10^3, order of magnitude 3; 8,000 rounds up to about 10^4, order 4. Going up one order means multiplying by ten. This lets you compare wildly different things quickly: a human (about 10^0 m tall) is some 15 orders of magnitude bigger than a proton (about 10^-15 m) and 26 orders smaller than the observable universe.
Order-of-magnitude thinking is a physicist's reflex for sanity-checking. If a calculation for the mass of a car comes out as 10^7 kg, you know at a glance it is a thousand times too big (cars are about 10^3 kg) even before hunting for the algebra slip. It is the coarse first look that catches gross errors.
Is 10 million people in a city plausible? A large city is order 10^6 to 10^7 people. So yes; but 10^9 (a billion) in one city would be off by orders of magnitude and clearly wrong.
Each order of magnitude is one factor of ten.
Same order of magnitude does not mean equal: 2 times 10^3 and 9 times 10^3 are both order 10^3 yet differ by more than four times. It is a coarse comparison, on purpose.