Introspection, in psychology, means to examine experience for its constituents, or elements. It was utilized by the first psychologist (Wilhelm Wundt) as a method of discovering the elements of conscious experience. Basically, Wundt showed visual stimuli to well-trained subjects and asked them what elements they were experiencing. The method proved to be unreliable as other researchers, using other subjects, discovered other elements than Wundt. Wundt's first student in America, Bradford Titchner, had just such a long-running disagreement with Wundt.
The common use of "introspection" today is to look inward in an effort toward self-understanding. While both the present-day and the historical uses of the term utilize observations of unknown veracity, it is important to note that psychology, historically, used the method to examine sensory phenomena to deduce elements of mental experience, not explanations for individual behavior.