Latin: “from the latter”.
In Epistemology the adjective describing beliefs or knowledge that depend upon previous experience (usually sensory experience).
For example, the idea of what a rose smells like, or what royal blue looks like is said to be ‘a posteriori’ because the existence of the idea is dependent on the experience of smelling a rose, or having seen the color royal blue.
Similarly, the proposition "Bill is a bachelor" is known to be true only if we have met Bill, and know that he is unmarried.
The Empirical Hypothesis (most famously articulated by John Locke in the Western philosophical tradition) asserts that since we are born without innate ideas (i.e., as a blank slate), all the ideas we have must be derived from experience.
Radical Empiricism maintains that since ALL knowledge is dependent on the ideas we have, and since all ideas are a posteriori, all knowledge must be a posteriori.