1590s, from Middle French hypothese and directly from Late Latin hypothesis, from Greek hypothesis "base, basis of an argument, supposition," literally "a placing under," from hypo- "under" (see sub-) + thesis "a placing, proposition" (see thesis). A term in logic; narrower scientific sense is from 1640s.
hypothesis hy·poth·e·sis (hī-pŏth'ĭ-sĭs)
n. pl. hy·poth·e·ses (-sēz')
A tentative explanation that accounts for a set of facts and can be tested by further investigation.
plur. hypotheses (heye-poth-uh-seez)
In science, a statement of a possible explanation for some natural phenomenon. A hypothesis is tested by drawing conclusions from it; if observation and experimentation show a conclusion to be false, the hypothesis must be false. (See scientific method and theory.)