Existentialism is a twentieth-century philosophy holding that human beings have no fixed essence or built-in purpose. A person exists first and must then define who they are through free choice, bearing full responsibility for those choices in a universe that offers no external guarantee of meaning. It is most closely tied to Jean-Paul Sartre and the postwar French thinkers around him, though its roots reach back to Kierkegaard and Nietzsche.
Sartre called it a sentence, not a gift: man is condemned to be free. There is no God to consult, no human nature to blame, no script written in advance. Whatever you become, you chose it, and you cannot hand the choice to anyone else. The weight of that, with no authority above you to share it, is what existentialists named anguish. The philosophy that told a bombed-out Europe it was radically free landed less like a liberation than like a verdict.