Most researchers agree that there is a continuum between well-definedness and ill-definedness. However, positioning a specific problem within this continuum is not always easy. To determine the degree of ill-definedness of a problem, in this paper, we propose a classification into the following five classes with respect to the size of the problems's solution space: 1) one single solution, 2) one solution strategy which can be implemented in different variants, 3) a limited number of alternative solution strategies which can be implemented in different ways, 4) a great variety of possible solution strategies where solutions can be verified, and 5) the correctness of a solution cannot be verified.