Sentence:  We devote a chapter to the binomial distribution not only because it is a mathematical model for an enormous variety of real life phenomena , but also because it has important properties that recur in many other <WSD>probability</WSD> models . 
Meaning of the word marked with <WSD> tags:  a measure of how likely it is that some event will occur; a number expressing the ratio of favorable cases to the whole number of cases possible