n.
a measure of how likely it is that some event will occur; a number expressing the ratio of favorable cases to the whole number of cases possible; "the probability that an unbiased coin will fall with the head up is 0.5"
n.
an unknown and unpredictable phenomenon that causes an event to result one way rather than another; "bad luck caused his downfall"; "we ran into each other by pure chance"
v.
come upon, as if by accident; meet with; "We find this idea in Plato"; "I happened upon the most wonderful bakery not very far from here"; "She chanced upon an interesting book in the bookstore the other day"