Final answer:
A posteriori probability is an empirical probability value determined after collecting data, which is in contrast to a priori probability that is deduced from reason alone.
Step-by-step explanation:
A posteriori probability refers to a probability value determined after collecting data. This type of probability is empirical, meaning it is based on and verifiable through observation and experience. For example, after observing that a medication led to improvement in 70 out of 100 patients, the a posteriori probability of effectiveness for that medication would be considered 70%. It contrasts with a priori probability, which is deduced before any empirical data is observed, typically based on logical reasoning or theoretical derivation.