Sentence:  More generally , suppose that an experiment consists of a number of independent trials , that each trial results in either a `` success '' or a `` non success '' ( `` failure '' ) , and that the <WSD>probability</WSD> of success remains constant from trial to trial . 
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