A disperser is a one-sided extractor. Where an extractor requires that every event gets the same probability under the uniform distribution and the extracted distribution, only the latter is required for a disperser. So for a disperser, an event we have:
Definition (Disperser): A -disperser is a function
such that for every distribution on with the support of the distribution is of size at least .
Read more about Disperser: Graph Theory, Other Meanings