probability
Pluralprobabilities
- 1The relative likelihood that a given event will happen.
Engineers calculated the probability of a part failing under stress.
Investors judge the probability of a downturn before shifting their money.
Similarlikelihood, chance, odds, risk
- 2The state or quality of being probable.
The probability of the bridge holding seemed reasonable to the inspectors.
Given the evidence, the probability of his innocence struck the jury as slim.
Similarlikelihood, chance
Oppositeimprobability
Note
Belief short of proof is what this sense first named: Latin probābilitās meant "credibility," from probābilis, "credible." Dryden writes that traditions "make not Truth but Probability," and Locke sets it against demonstration, calling it the appearance of agreement between ideas. It still sits naturally beside judgments of evidence: the probability of a claim, of a witness's account, of someone's innocence.
- 3An outcome that has become likely; a real prospect.
A second wave of layoffs became a clear probability after the report.
With supplies running low, rationing turned into a probability nobody welcomed.
Similarlikelihood, prospect, possibility
Oppositeimprobability
Note
Here likely outcomes become things one can count and list, and the plural carries an idiom of its own: "the probabilities are that…" Letitia Elizabeth Landon's Ethel Churchill (1837) warns that "the probabilities are, that you will waste the rich treasure of your affection." In the singular the noun takes an article and often an adjective: a clear probability, a probability nobody welcomed.
- 4A precise measure of how likely an event is, expressed as a number from 0 (impossible) to 1 (certain).
The probability of rolling a six on a fair die is one sixth.
Assigning a probability of 0.5 to each side reflects a balanced coin.
Similarchance
Note
Mathematics turns likelihood into a measure. Every event gets a value from 0, for what cannot happen, to 1, for what must, and the standard notation is P(A), read "the probability of A." Unlike the everyday sense of likelihood, this one always demands an exact value, written as a fraction or a decimal: one sixth for rolling a six, 0.5 for heads on a fair coin.
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Origin
The word came into English from Middle French probabilité, which came from Latin probābilitās ('probability, credibility'), derived from probābilis ('probable, credible').