Chance: What happens in reality but was not caused or not certain to happen.
A related usage of "chance" is as "probability", but the latter term is more comprehensive.
The main reason why scientifically inclined persons believe in the existence of chance in reality is that Quantum Mechanics is a highly confirmed part of physics, that led to a lot of succesful technology, and so must be adequate in some ways, and is firmly based on the assumption that there really are chance processes in nature - things that happen without any prior cause, that cannot be individually foretold, except that in the mass they happen according to some statistical distribution that can be experimentally found.
As defined, chance events may also comprise personal choices of people, since these are usually not certain until they happen, though they may be fairly predicted with considerable probability if one knows the person's character, and though the person who made the choice may firmly believe he personally determined by that choice part of his fate, for reasons of his own, within his own discretion.
All of this may be true, but then whatever happens still will be a chance event for others, and presumably also for the one who choose to do this rather than that, even when this was done deliberately and with conviction, in as much as choices one can make are not certain until they have been made.
Note also that chance, as defined, may be pragmatical, practical or unavoidable, in as much as much that happens, even what may happen perfectly deterministically and predictably if one had complete information, just cannot be dealt with other as a chance process, event or outcome, of which one has at best uncertain and incomplete information.