How likely a thing is, stated as a number. Probability describes a state of knowledge as much as a state of the world.
Latin probabilitas (credibility, provableness), from probabilis (worthy of approval, credible), from probare (to test, to approve), from probus (good, upright); a probability was first a measure of how believable a claim was, only later a measure of how likely an event is.
How likely a thing is, stated as a number rather than a feeling. The root's older sense is worth keeping: a probability is also a measure of how much confidence the claim deserves, not only a fact about the world.