Coincidence is defined as two or more events occurring at the same time with no apparent connection between them. These events usually share some common quality that makes it seem odd that they should happen together without a cause and effect type of relationship. This is different from probability, which is defined as a measure of how likely it is for a given event to occur. A probability is usually well known. For example, the probability of rolling a 4 given a fair six-sided die is 1/6, or approximately 0.1667. Coincidences are not predictable given that there is no discernible connection between them.

Probability ranges from zero to one, and can never fall outside of this range. A probability of zero means that the even can never take place, so any negative value of probability has no meaning. A probability of 0.2 means that there is a 20 percent chance for the event to take place and an 80 percent chance that the event will not take place. A probability of 1 means that the event will take place with certainty, and thus a probability greater than 1 is also meaningless.

Probability is used for business to provide a perspective on risk versus reward for future decisions. If the purchase of a $100 machine has a 25% chance to generate $1000, then that is a good investment since every four times the machine is purchased the expected profit is $600.