The plausible range around a study's result.
A Confidence Interval is a range of values that is likely to contain the true effect, given the data — a 95% CI is the range you would expect to capture the true value 95% of the time if the study were repeated. A wide interval signals uncertainty; an interval that crosses 'no effect' means the result is not statistically clear. It conveys precision in a way a single number cannot.