probability (n.)

mid-15c., probabilite, "likelihood of being realized, appearance of truth, quality of being probable," from Old French probabilite (14c.) and directly from Latin probabilitatem (nominative probabilitas) "credibility, probability," from probabilis (see probable).

Meaning "something likely to be true" is from 1570s; mathematical sense is from 1718, "frequency with which a proposition similar to the one in question is found true in the course of experience."

In weather forecasting, probabilities was used in U.S. from 1869 and adopted in the official weather forecasts of the United States Signal Service; hence Old Probabilities, a humorous name for the chief signal officer of the Signal Service Bureau (by 1875).

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Definitions of probability from WordNet

probability (n.)
a measure of how likely it is that some event will occur; a number expressing the ratio of favorable cases to the whole number of cases possible;
the probability that an unbiased coin will fall with the head up is 0.5
Synonyms: chance
probability (n.)
the quality of being probable; a probable event or the most probable event;
for a while mutiny seemed a probability
going by past experience there was a high probability that the visitors were lost