Jack has experienced recurrent episodes of major depressive episodes. In the intervals between the episodes, he does not seem to return to "normal." In fact, during those periods, he has been diagnosed as having the symptoms of persistent depressive disorder (dysthymia). Jack's condition is referred to as

Double depression.

Explanation:

Double depression is complicated form of mental illness that is dysthymia which is also known as dysthymic disorder which is a psychiatric illness. It is characterized by the depressed mood with couple of symptoms of depression that are low confidence or self esteem, tiredness, sadness and other.

In the case of Jack who has diagnosed for the dysthymia in time periods in between the major depressive episodes which indicates that it is the case of the double depression.