Order of magnitude
"Order of" is often meant as "it's probably more or less right / at most a small factor off".
"Order of magnitude" can be meant specifically as a factor ten
- so nothing more than "that number probably has the right amount of zeroes on it", which can be much more off
Some people make clearer distinction between the two than others.
Ballpark figure is often the first, but not always.
- The original meaning seems to be commentators eyeballing the amount of visitors based on filled and empty seats, which would probably always be within a few dozen percent and frequently closer
- but e.g. people figuring distances on a large scale often has proportionally larger error