Definitions
1.nounUsed loosely, a tentative conjecture explaining an observation, phenomenon or scientific problem that can be tested by further observation, investigation and/or experimentation. As a scientific term of art, see the attached quotation. Compare to theory, and quotation given there.
2.noun(general) An assumption taken to be true for the purpose of argument or investigation.
3.noun(grammar) The antecedent of a conditional statement.