None of the great existentialist tomes contains the word 'existentialism'. Reports on its origin differ, but it seems to have been coined towards the end of World War II by the French philosopher Gabriel Marcel as a label for the currently emerging ideas of Jean-Paul Sartre and his close friend Simone de Beauvoir. According to the latter, neither of them initially appreciated this baptism.

During a discussion organized during the summer [of
1945], Sartre had refused to allow Gabriel Marcel to
apply this [word] to him: '. . . I don't even know what
existentialism is'. I shared his irritation . . . But our
protests were in vain. In the end, we took the epithet
. . . and used it for our own purposes.

Sartre, in fact, 'took' it rather quickly, for in the autumn of that year he delivered the lecture which became the most widely read of existentialist writings, Existentialism and Humanism...