What Is It?
"An ongoing or upcoming concern that has a significant probability of adversely afffcting the success of major milestones.."
The Rational Unified Process Made Easy Per Kroll, Phillipe Krutchen. Addison Wesley

"Risk is the possibility of suffering loss, injury, disadvantage, or destruction."
[Webster's Third New International Dictionary 1981]

"Risk is the potential for realization of unwanted negative consequences of an event."
[Rowe, William D. An Anatomy of Risk 1988]

"Risk is the measure of the probability and severity of adverse effects."
[Lowrance, William W. Of Acceptable Risk 1976]