What does probability mean?
Probability is the degree of likelihood that an event will happen or that a statement is true. It appears in ordinary conversation when people describe uncertainty: a forecast may indicate a high probability of storms, while a cautious plan reflects a low probability of success. In mathematics, probability gives random outcomes a numerical measure, usually ranging from zero for an impossible event to one for a certain event. The concept is useful because it does not claim that an uncertain outcome is predictable; instead, it expresses how likely that outcome is under specified conditions. Probability can therefore support judgments in science, economics, law, insurance, weather forecasting, and everyday decision-making, while also helping distinguish confidence from certainty.
nounThe likelihood that an event will occur, often expressed as a proportion, percentage, or mathematical value between zero and one. In general use, probability means the possibility that something is true, will happen, or will be the case.
- The likelihood that an event will occur or that a proposition is true.
- In mathematics, the numerical measure of the likelihood of an event, usually expressed as a value between zero and one.
"There is a high probability of rain this afternoon."
"The probability of catching the early train was too low to rely on."
"In probability theory, each possible result of a random experiment is assigned a likelihood value."
The regular plural is “probabilities,” with the final syllable changing from -ity to -ities.
"The model compares several probabilities before selecting the most likely outcome."
Probability turns uncertainty into a number, letting us reason about chance even when the future remains unknown.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does probability come from?
The word comes through Latin from probare, meaning “to test” or “to approve,” and the related adjective probabilis, meaning “credible” or “likely.” The suffix -ity is used here in the sense of condition or quality, so probability originally referred to the quality of being probable. Over time, the word developed into both a general term for likelihood and a precise mathematical concept.
Related word forms
How probability is actually used
In everyday use, probability often means degree of likelihood and may be paired with words such as high, low, and unlikely. In mathematical contexts, it has a precise technical meaning and is typically represented by a value from zero to one.
Easily confused with probability
Possibility refers broadly to whether something can happen or be true, while probability describes the likelihood of an event, often on a numerical or graduated scale.