(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.