hypothesis UK /haɪˈpɒθɪsɪs/US [haɪˈpʰɑθəsɪs] noun3 •A tentative conjecture explaining an observation, phenomenon or scientific problem and that can be tested by further observation, investigation, or experimentation. •An assumption taken to be true for the purpose of argument or investigation. •The antecedent of a conditional statement. formshypotheses