means The branch of mathematics that measures how likely something is to happen, expressed as a number between impossible (0) and certain (1).

from From Latin 'probabilis,' meaning 'provable' or 'credible,' rooted in 'probare,' 'to test, prove, approve' — the same source that gives us 'probe' and 'prove.' For centuries it simply meant 'worthy of belief'; the precise mathematical sense was forged in the 17th century out of correspondence between Pascal and Fermat puzzling over gambling problems, when 'likely to be true' hardened into 'measurably likely.'