APA Dictionary of Psychology

n. the power to which the expected value of a random variable is raised. Thus, E(x^k) is the kth moment of x. The first moment is usually the mean of a variable, the second moment refers to variance, the third moment relates to skewness, and the fourth moment concerns kurtosis. Knowing each of these moments provides a complete picture of the distribution for a set of scores: A researcher knows the center point of the data, how spread out the values are, whether they are lopsided, and whether they are peaked (leptokurtic) or flat (platykurtic).