Shannon's definition is basically unique in that it is the only such one that has certain properties: it is determined entirely by the probability distribution of the data source, it is additive for independent sources, it is maximized at the uniform distribution, it is minimized (and equal to zero) when there is 100% probability of only one event occurring, and it obeys a certain derived version of the chain rule of probability. Axiomatic derivations of entropy are explained further below on the page.