A risk is a situation where the outcome is unknown, more specifically carrying some chance of loss or injury.
In many of life's situations, the only possibilities of gain include some risk, such as investing in a business or trusting another person. "Risk management" is the art and science of minimizing the risk inherent in a given endeavor. Colloquially, "risky behavior" means activities that are perceived as having a very high risk to benefit ratio, especially if the risks can easily be reduced by simple means.
Parts of game theory study the risk versus reward aspect of artificial and real life situations.
 Other uses
Risk is also a board game where you try to take over the world. If you don't have a lot of time to kill, try playing a quicker game such as Monopoly.
 See also