1. hypothesis; supposition; assumption; tentative theory
Wikipedia definition
2. HypothesisA hypothesis (from Greek ὑπόθεσις; plural hypotheses) is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot satisfactorily be explained with the available scientific theories.
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