Although a layman may treat the terms suggested in another answer as interchangeable, they are not totally so. About Education has an article explaining that more precise definitions are demanded in scientific registers:
Words have precise meanings in science. For example, 'theory', 'law',
and 'hypothesis' don't all mean the same thing. Outside of science,
you might say something is 'just a theory', meaning it's supposition
that may or may not be true. In science, a theory is an explanation
that generally is accepted to be true. Here's a closer look at these
important ... terms.
A hypothesis is an educated guess, based on observation.
It's a prediction of cause and effect. Usually, a hypothesis can be
supported or refuted through experimentation or more observation. A
hypothesis can be disproven, but not proven to be true.
A scientific theory summarizes a hypothesis or group of hypotheses
that have been supported with repeated testing.
A theory is valid as long as there is no evidence to dispute it.
Therefore, theories can be disproven.
'Assumption' would not normally be used to identify a hypothesis of major significance, and '[educated] guess' never.
'Conjecture' and 'postulate' are closer to 'hypothesis', but (and even scientific usage is not as precise as scientists might like to imagine) the order conjecture ...... postulate ... hypothesis probably indicates increasing confidence in the truth of the statement.
And it would be a mistake to break up set compounds such as 'Greek hypothesis', 'Riemann hypothesis', 'Goldbach's conjecture', 'Twin prime conjecture'.