Existentialism, in Sartre’s seminal work, Being and Nothingness, is a philosophical exploration of the implications of the idea that “existence precedes essence”—in other words that humanity is not predesigned by a Creator, but that humanity invents and defines itself by its choices. His physical model or analogy is the paper-cutter: someone put the parts together (hinge, blade, handle, etc.) to do a specific job—to cut paper—and philosophical questions can be asked based on how well it does that job. The controversial part of Sartre’s inquiry (besides the implication that there is no Creator) is how such a notion alters our notions of right and wrong action. A second, much shorter and more comprehensible Sartre study, called Existential and Human Emotion, addresses how we apply existentialism to everyday decisions.
In literature, twentieth-century novelists and dramatists took existentialism as a starting point of their socio/philosophical inquiries, so Camus, Gide, Ionesco, Beckett, and dozens of others are now called “existential writers,” meaning that they fictionalized or dramatized the very consequences that Sartre was suggesting. (Sartre, too, wrote an existential drama, The Flies).
Existentialism is a philosophical thought.