A tentative explanation that accounts for a set of facts and can be tested by further investigation; a theory [ISERN]
The hypothesis is the tentative theory of supposition that you think explains the behaviour you want to explore. Wherever possible, you should try to state your hypothesis in quantifiable terms, so that it is easy to tell whether the hypothesis is confirmed or refuted [Fenton&Pfleeger 1996].
See also: research hypothesis, statistical hypothesis.