/ ˌsʌpəˈzɪʃn; NAmE / noun ( formal )
1.
[ C ] supposition (that ... ) an idea that you think is true although you may not be able to prove it
SYN assumption :
The police are working on the supposition that he was murdered.
2.
[ U ] the act of believing or claiming that sth is true even though it cannot be proved :
The report is based entirely on supposition.
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WORD ORIGIN
late Middle English (as a term in scholastic logic): from Old French , or from late Latin suppositio(n-) (translating Greek hupothesis hypothesis), from the verb supponere , from sub- from below + ponere to place.