On 29 October 1945, in newly-liberated Paris, Jean-Paul Sartre delivered a public lecture titled L'existentialisme est un humanisme — Existentialism is a Humanism. The hall was so crowded that people fainted. The text (published 1946) became one of the most-read philosophical works of the twentieth century and gave existentialism — until then a fringe academic position associated with the German phenomenology tradition — its public face. Sartre's slogan, l'existence précède l'essence (existence precedes essence), captured the position: there is no human nature given in advance; we make ourselves by our choices. We are condemned to be free.
Existentialism is best understood as a family of positions sharing a characteristic emphasis rather than a fixed doctrine. The central thesis is that existence precedes essence — for human beings only, since the chair is still chair-shaped before it exists. Traditional metaphysics, from Plato through medieval Christianity, supposes that things have essences that precede and govern their existence; existentialism reverses the order for people. We are first thrown into existence, and only by our choices do we become what we are. The position implies radical freedom (no external authority can ground our choices for us) and radical responsibility (we must own those choices). Confronting that freedom produces Angst — Heidegger's term, Kierkegaard's Angest — not fear of any specific threat but the dizzying recognition of no external grounding. Authenticity is the existentialist response: acknowledging the contingency of one's identity and taking responsibility for it. Its opposite, what Sartre calls bad faith, is the lie one tells oneself to deny one's own freedom — usually by treating an externally-given role as necessity rather than choice.
Existentialism is not nihilism. Sartre, Simone de Beauvoir, and Albert Camus all argue that we are thrown into a situation — historical, social, embodied — and must engage with it. Sartre joins and later breaks from the Communist Party; Camus joins the French Resistance; de Beauvoir writes The Second Sex (1949), one of the founding texts of modern feminism. The intellectual lineage runs through Søren Kierkegaard (the leap of faith, anxiety as the dizziness of freedom), Friedrich Nietzsche (the death of God, the Übermensch), Edmund Husserl (phenomenology), and most consequentially Martin Heidegger (Being and Time, 1927) — whose Nazi-era affiliation has shadowed the reception of his philosophy ever since and remains unresolved.
Existentialism's cultural moment, roughly 1945 to 1965, has passed, but the philosophical reception persists. Phenomenology — the broader tradition existentialism emerged from — remains alive in philosophy of mind (embodied cognition owes much to Maurice Merleau-Ponty), in cognitive science, and in psychiatric phenomenology. Existential psychotherapy (Yalom, May) is a recognized therapeutic tradition; Viktor Frankl's logotherapy, articulated in Man's Search for Meaning (1946), has sold over sixteen million copies. The themes the framework named — climate anxiety, AI anxiety, the meaning crisis — are the contemporary forms of existential confrontation, recognizably continuous with the postwar ones.