sup ‧ po ‧ si ‧ tion /ˌsʌpəˈzɪʃ ə n/ BrE AmE noun [uncountable and countable]
[ Word Family: verb : ↑ suppose , ↑ presuppose ; noun : ↑ supposition , ↑ presupposition ; adverb : ↑ supposedly ; adjective : ↑ supposed ]
something that you think is true, even though you are not certain and cannot prove it:
His version of events is pure supposition.
supposition (that)
The police are acting on the supposition that she took the money.