A hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, ?p?t????a? – 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 satisfactorily be 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. A working hypothesis is a provisionally accepted hypothesis.

hy·poth·e·sis (hi-poth'i-sis)

noun pl. hypotheses hy·poth·e·ses (-sez?)
A tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation.
Something taken to be true for the purpose of argument or investigation; an assumption.
The antecedent of a conditional statement.