A confidence interval is an interval that brackets a sample estimate that quantifies uncertainty around this estimate. Since there are a variety of samples that might be drawn from a population, there are likewise a variety of confidence intervals that might be imagined for a given population parameter (though with the observed data you can see only one of them). A 95% confidence interval, for example, is one interval in a set of intervals. The property of this set of intervals is that 95% of the intervals in this set contain the population parameter. Likewise, a 90% confidence interval for the population mean is an interval which belongs to a set of intervals, 90% of which contain the population mean.