one-tailed probability

n. In inferential statistics, the probability that one would find a result as extreme as or more extreme than that actually found in one direction from the mean if the null hypothesis were true. The one-tailed (1-tailed) probability refers to the appearance of one end of a probability distribution far from the mean which appears similar to a tail. Since the area under probability distributions, such as the normal curve, in one end or tail is half that in the two tails combined, then the probability of obtaining an extreme result is half that of a 2-tailed probability, and thus a 1-tailed test has twice the statistical power of a 2-tailed test in symmetrical probability distributions.