- 1[countable] supposition (that…) an idea that you think is true although you may not be able to prove it synonym assumption The police are working on the supposition that he was murdered. All the evidence appears to support this supposition. It seems a reasonable supposition.
- 2[uncountable] the act of believing or claiming that something is true even though it cannot be proved The report is based entirely on supposition. What happened next is a matter of supposition.