In this sense it is a type of nostalgia, in the original sense of that word. At other times it may seem as a longing for a someone or even a something. But the majority of people who experience it are not conscious of what or who the longed for object may be. Indeed, the longing is of such profundity and intensity that the subject may immediately be only aware of the emotion itself and not cognizant that there is a something longed for.