Problem-based learning can be described as:
a way of constructing and teaching courses using problems as the stimulus and focus for learner activity. It is not simply the addition of problem-solving activities to otherwise discipline centered curricula, but a way of conceiving of the curriculum which is centered around key problems in professional practice. (Boud and Feletti 1991, 14)
Boud, D., and G. Feletti, eds. 1991. The challenge of problem-based learning. New York: St. Martin’s Press.