Latin terms "a priori" and "a posteriori" are the philosophical phrase that Immanuel Kant popularized and made possible their widespread use in his work "Critique of Pure Reason". Much earlier, before this popularization by Kant, these terms are used in Latin translations of Euclid's Elements, about 300 BCE.
These terms are used in relation to reasoning and conclusion, which distinguish "premise" conclusions from those that come after, conclusions based on sensory observations.
A Priori conclusions are independent of experience, for example 4 + 5 = 9, and result from pure reason. We do not need experience, but reason.
A posteriori conclusions come from experience and empirical evidence. They necessarily imply a general conclusion, for example, Civil War in America ended in 1865. If this is true we must conclude on the basis of an empirical fact that can not be recognized only by reason, and comes a posteriori.
So, "a posteriori" phenomenon is the answer.