two-tailed probability

n. In inferential statistics, the probability that one would find a result as extreme or more extreme than that actually found if the null hypothesis were true. The description two-tailed (2-tailed) refers to the appearance of the ends of a probability distribution far from the mean which appear 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 twice that of a 1-tailed probability and so a 2-tailed test has half the statistical power of a 1-tailed test in probability distributions.