A wicked problem is one for which each attempt to create a solution changes the understanding of the problem. Wicked problems cannot be solved in a traditional linear fashion, because the problem definition evolves as new possible solutions are considered and/or implemented. The term was originally coined by Horst Rittel.
Wicked problems always occur in a social context -- the wickedness of the problem reflects the diversity among the stakeholders in the problem.
Most projects in organizations -- and virtually all technology-related projects these days -- are about wicked problems. Indeed, it is the social complexity of these problems, not their technical complexity, that overwhelms most current problem solving and project management approaches