In 1973, Rittel, H.W.J. & Webber, M.M. coined the term "Wicked Problem" in Dilemmas in a general theory of planning, Policy Sci (1973) 4: 155. 
The RESIN project  gives the following definition:
Wicked problems are commonly found within complex environments, where constantly evolving interdependencies make it difficult to define problems precisely. (Here, “wicked” is used in the sense of “complex” or “challenging”, as opposed to “ethically deplorable”.
- doi:10.1007/BF01405730 doi:10.1007/BF01405730
- Wijnmalen, D., V. Kamphuis, R. Willems. Decision Support. State of the art report 6, The Resin Project
- Foresight: A Glossary, Civil Service College, Singapore