Existentialism is the philosophical concept that existence precedes essence. This is contrasted with the Platonic idea of the Forms, in which essence precedes existence. (According to Platonic ideas, there is a form of every tangible object, and the object itself is simply an imperfect copy of the form. This idea can also be applied to intangible concepts such as that of justice.)
To put this in quasi-Christian terms, an existentialist does not think they were born with an immortal "soul" - if there is any such attribute to a person, it is developed throughout life.
The origins of modern existential thought are developed and explored in a thread of philosophical and literary works from Søren Kierkegaard (who famously declared that "Truth is Subjectivity") and Friedrich Nietzsche (who famously declared that "God is dead"), through Martin Heidegger and Karl Jaspers, to the French writers Jean-Paul Sartre, Albert Camus, and Simone de Beauvoir, and playwrights Eugène Ionesco and Samuel Beckett (and Sartre again).
 In the 20th century
Existentialism peaked in popularity, both in serious philosophical thought and popular culture influence, from the 1950s to the 1970s. In terms of popular culture influence, a lot of novels and films during the period formed their premises around what the writers thought existentialism was about:
- life is meaningless
- you create your own reality
- we are doomed to be free
- being and experiencing are preferable to following a moral code or some purpose in life
- human potential is unlimited even though there is no ultimate meaning to it.
These lines of thought make existentialism somewhat related to (although not the same thing as) nihilism and egoism. Existentialism in some ways was a philosophy ready-made for the post-World War II era, during which people struggled to make sense of the evils that had just occurred and tended to hold grand purposes in life in disdain (seen as having led, among other things, to communism and fascism). After the 1970s, academic philosophers moved away from existentialism into other influences such as postmodernism.
 Further reading
- Irrational Man: A Study in Existential Philosophy, (1958), Barrett, William, Doubleday (Anchor paperback 1962), New York. ISBN-10: 0385031386 - A good overview of the history of existential thought.
- Existentialism from Dostoevsky to Sartre, (1975), Walter Kaufmann (editor), Plume/Meridian. ISBN-10: 0452009308 - by the man who brought us a great series of translations of Nietzsche's works.