It means "from what goes before". A priori is an argument which uses a truth or generally accepted principle to deduce the effect of a particular set of facts.

If George V reigned at least four days, then he reigned more than three days.
This is something that one knows a priori, because it expresses a statement that one can derive by reason alone.

See, a posteriori, a fortiori.

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A fortiori A posteriori A priori Ab initio abatement