Probability Mass Function 

For discrete random variables, the PMF is also called the probability distribution. Thus, when asked to find the probability distribution of a discrete random variable XX, we can do this by finding its PMF. The phrase distribution function is usually reserved exclusively for the cumulative distribution function CDF (as defined later in the book). The word distribution, on the other hand, in this book is used in a broader sense and could refer to PMF, probability density function (PDF), or CDF.