A number that is given with very large uncertainties, such that it might be ten or a hundred (or more) times larger or smaller than the value given. Astrophysics is famous for having 'errors in the exponent'
..."We estimate the minimum viable population to be somewhere between 100 and 5000" (extremes from the wikipedia article)
... "in our universe protons decay with a time scale in the range 10^33 - 10^45 years"
These quantities don't have an error in the value, they have an error in the exponent.