A hypothesis is a proposed explanation for a phenomenon.
As an example, based on the quality of Wikipedia articles, one could form a hypothesis that Wikipedia articles can only be edited by highly qualified professors with multiple Ph.Ds. It can be considered a hypothesis, as it is falsifiable; it can be falsified by noticing that anyone can edit Wikipedia articles, using the 'edit page' link on all pages. An experiment in this regard would be to simply click that link, edit the page, and save. If the replaced page appears, and you do not have these multiple Ph.Ds, your hypothesis is falsified, and the experiment ends.
See Philosophiae Naturalis Principia Mathematica for Newton's position on hypotheses, "Hypotheses non fingo" : "I feign no hypotheses" .
See also Statistical hypothesis testing.
 Isaac Newton, Principia Mathematica. A New Translation by I. Bernard Cohen and Anne Whitman, translators. University of California Press 1999 ISBN 0-520-08817-4