Sentence:  When each number of successes x is paired with its probability of occurrence * * f , the set of pairs * * f , is a probability <WSD>function</WSD> called a binomial distribution . 
Meaning of the word marked with <WSD> tags:  (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)