It stands for confidence interval; a statistical measure that represents the mean of an estimate plus and minus the variation in that estimate. It produces or defines a range of values within which an estimate is expected to fall, and within a certain level of confidence.

In frequentist statistics, a confidence interval is an range which is expected to typically contain the parameter being estimated. At a given a confidence level, a confidence interval (CI) is a random interval within which the parameter being estimated falls for a certain percentage of the time.

A confidence interval represents the probability that a population parameter (a measurement) will fall within a certain range defined by two set values (upper and lower). It tells how precise the measurement is.