Used 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.
User Defined Definitions
"a proposal intended to explain certain facts or observations"
"a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena"
"a message expressing an opinion based on incomplete evidence"