Existentialism is notoriously hard to define.
It is a label that has been rejected by some of the philosophers to whom it has been attached. In Existentialism and Humanism, Sartre states that the common idea that binds existentialist thinkers is that 'existence precedes essence'. Even in this Hiedegger claims Sartre misunderstood him. Existentialism is a product of it's historical, social, political and economic context and that it is only a partial analysis of existentialist thought that attempts to divorce it from this. While the themes that help us to decide whether a text is 'existential' or not recur again and again, the specific concerns of the thought will vary due to the context within which they were written.