Uncertainty: Lack of certainty - what is neither determinedly true nor determinedly false, and hence in some sense and to some degree probable.
There are several ways and senses in which a statement can be quite determinedly uncertain, and standard probability theory is one good approach to such statements. There are others, and also quite a few different kinds and sorts of probability theory.
Note one very fundamental case of uncertainty: Concerning contingent statements about the future. This differs from uncertainty about the past in that what is in the past definitely has happened or definitely has not happened, whereas what is contingent and in the future so far neither definitely has happened nor definitely has not happened, even though in either case, whether one is considering the past or the future, one may not know which alternative eventually will turn out to be the case.