DEFINITION of 'Prior Probability'
The probability that an event will reflect established beliefs about the event before the arrival of new evidence or information. Prior probabilities are the original probabilities of an outcome, which be will updated with new information to create posterior probabilities.
INVESTOPEDIA EXPLAINS 'Prior Probability'
Prior probabilities represent what we originally believed before new evidence is uncovered. New information is used to produce updated probabilities and is a more accurate measure of a potential outcome. For example, three acres of land have the labels A, B and C. One acre has reserves of oil below its surface, while the other two do not. The probability of oil being on acre C is one third, or 0.333. A drilling test is conducted on acre B, and the results indicate that no oil is present at the location. Since acres A and C are the only candidates for oil reserves, the prior probability of 0.333 becomes 0.5, as each acre has one out of two chances.

Bayes' Theorem
A formula for determining conditional probability named after ... 
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A Priori Probability
Probability calculated by logically examining existing information. ... 
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