A scientist is not merely concerned to accumulate such facts as can be discovered by observation. A proposition or set of propositions set forth as an explanation for the occurrence of some specified group of phenomena either asserted merely as a provisional conjecture to guide investigation working hypothesis or accepted as highly probable in the light of established facts. Outside science a theory or guess can also be called a hypothesis.
It may be easier to think of it as a nullifiable hypothesis or one that the researcher seeks to nullify.
One of the earliest uses of the word in this sense was in geometry. In spite of induction and computer science tags an induction hypothesis is different a little. The null hypothesis is that there is no change in fatality rate while the alternative hypothesis is that the fatality rate has decreased. The null hypothesis is useful because it can be tested and found to be false which then implies that there is a relationship between the observed data.