Having a good reason to believe that something is true means that it is presumptive — you could call a person you assume will be nominated for school president the "presumptive nominee."

After an election, when the votes are still being counted but it's clear who the winner will be, reporters often talk about that person as the presumptive winner. And if a doctor were fairly certain you had strep throat, she might call it her presumptive diagnosis. The Latin root word is praesumptionem, which means "confidence or audacity."

Definitions of presumptive

adj having a reasonable basis for belief or acceptance

“the presumptive heir (or heir apparent)”
likely, probable
likely but not certain to be or become true or real

adj affording reasonable grounds for belief or acceptance

presumptive evidence”
“a strong presumptive case is made out”
believable, credible
capable of being believed

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