A hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, hypotithenai meaning to put under or to suppose. For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot be satisfactorily explained with the available scientific theories. Even though the words hypothesis and theory are often used synonymously in common and informal usage, a scientific hypothesis is not the same as a scientific theory – although the difference is sometimes more one of degree than of principle.