The existential vacuum is an important concept in Man's Search for Meaning. Frankl defines it as the result of a twofold loss. The first loss is very ancient: when our ancestors first became human, they lost some of the basic instincts which give security to an animal. The second is more recent: we have lost many of the human traditions which used to tell us what we ought to do. Often, we do not even know what we want to do, with the result that we either do what everyone else does (conformism) or are forced to obey orders (totalitarianism).

When Frankl first introduces the term, he says that many of his patients complain of

the feeling of the total and ultimate meaninglessness of their lives. They lack the awareness of a meaning worth living for. They are haunted by the experience of their inner emptiness, a void within themselves; they are caught in that situation which I have called the "existential vacuum."

Frankl later remarks that in contemporary life, "The existential vacuum manifests itself mainly in a state of boredom." He discusses a condition he calls "Sunday neurosis." After a busy week, people are finally left alone with themselves and have no idea what to do. The void within them becomes clear. Frankl adds,

Not a few cases of suicide can be traced back to this existential vacuum. Such widespread phenomena as depression, aggression and addiction are not understandable unless we recognize the existential vacuum underlying them. This is also true of the crises of pensioners and aging people.

This is one of the few times when Frankl also specifically mentions existential crisis, by which he means the point at which the patient is thrown into turmoil by a sudden confrontation with the vacuum. He writes of the doctor's duty "to pilot the patient through his existential crises of growth and development" rather than simply prescribing drugs. He also discusses the sudden existential crisis often experienced by a prisoner in the concentration camps:

With his loss of belief in the future, he also lost his spiritual hold; he let himself decline and became subject to mental and physical decay. Usually this happened quite suddenly, in the form of a crisis, the symptoms of which were familiar to the experienced camp inmate.