A Wicked Problem, roughly speaking, has a number of properties:

1. The problem is not understood until after the formulation of a solution.

2. Wicked problems have no stopping rule.

3. Solutions to wicked problems are not right or wrong.

4. Every wicked problem is essentially novel and unique.

5. Every solution to a wicked problem is a 'one shot operation'

6. Wicked problems have no given alternative solutions.

Software development is largely a Wicked Problem, or in headline terms:

It is faster to create a solution to the problem than to estimate how long it will take to create that solution with any degree of accuracy, and the estimate will still be wrong.

http://en.wikipedia.org/wiki/Wicked_problem

http://www.poppendieck.com/wicked.htm