(STATISTICS) a range of values for which you are x percent confident contains the correct answer. Answers to a statistical question which are ranged from the lowest likely value to the highest; answers outside of this range are highly unlikely.
Presupposes that you are estimating a value based on sample data, and the sample data has a genuinely random variance.
Usually the confidence interval is for a 95% confidence, meaning there is only a 5% probability that the true value is OUTSIDE the interval.
ANNA: I've been driving your car for about a month.
JAMES: And what you think is the MPG?
ANNA: It's probably about 25 MPG, with a 95% confidence interval of 19.5 to 32 MPG.
JAMES: Holy cow! Any Prussians in your family tree?