The distribution function or the density [pmf for a discrete random variable] completely characterizes the behavior of a random variable X. Frequently, however, we need a more concise description such as a single number or a few numbers, rather than an entire function. One such number is the expectation or the mean, denoted by . Similarly, the median, which is defined as any number x such that and , and the mode, defined as the number x, for which or attains its maximum, are two other quantities sometimes used to describe a random variable X. The mean, median, and mode are often called measures of central tendency of a random variable X.