noun (formal)
1 [C] ~ (that...) an idea that you think is true although you may not be able to prove it
SYNASSUMPTION:
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 [U] the act of believing or claiming that sth is true even though it cannot be proved:
The report is based entirely on supposition. * What happened next is a matter of supposition.
