First of all that is not the definition of probability: probability is not an "event"- although I am sure the definition refers to events. I've never heard of a definition that did not "apply to all applications".
And, to satisfy my curiosity, what does this have to do with differential equations?
The standard definition of "probability" requires that "P(n)= probability that even n happens" be a number from 0 to 1. Thus the largest and probability can be is 1.
Sometimes probability is expressed as a percentage: 0 to 100% is, of course, 0 to 1.
Sometimes you will see "odds" as in "50 to 1 odds against" but that is not "probability". If the odds an event will happen are "a to b against" then the probability that it will happen is b/(a+b) which is always between 0 and 1.