A tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena WordNet 3.0 & Open English WordNet

A hypothesis is a proposed explanation for a phenomenon. Wikipedia

Proposed explanation for a phenomenon. Wikipedia Disambiguation

Especially one too vague to be formally stated or verified but useful as a working convention Wikipedia Disambiguation

A tentative conjecture explaining an observation, phenomenon, or scientific problem that can be tested by further observation, investigation, and/or experimentation. OmegaWiki

A thing possible; that which may take place or come into being. Wiktionary

A thing possible. Wiktionary (translation)

A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc. Wiktionary