Why did Camus reject the existentialist label, and how is the absurd different from nihilism?

Camus rejected the existentialist label because, in his own account, his roots lay elsewhere and his philosophy disagreed with existentialism on key points. And the absurd differs from nihilism because it is not just the claim that nothing matters: it is the conflict between our need for meaning and a world that gives none. Camus is routinely categorised as an existentialist, a term he rejected on several occasions.

Camus said his own philosophical origins lay in ancient Greek philosophy, Nietzsche and 17th-century moralists, whereas existentialism arose from thinkers such as Kierkegaard, Jaspers and Heidegger. He even said that The Myth of Sisyphus was a criticism of various aspects of existentialism. His critique was mostly aimed at Sartre's version, and he thought that the importance Marx and Sartre gave to history was incompatible with his belief in human freedom. Scholars add other explanations: David Sherman and others suggest the rivalry between Sartre and Camus played a part, and David Simpson argues that Camus's humanism and belief in human nature set him apart from the idea that existence precedes essence.

The friendship between the two men ended publicly. Sartre and de Beauvoir stayed friends with Camus until 1951, the year of The Rebel, a book that attacked totalitarian communism and brought about the final split. Even so, when Camus died, Sartre read a eulogy praising his 'stubborn humanism'.

On the other question: absurdism and nihilism share the belief that life is meaningless, but absurdism is interested in the conflict between the human desire for meaning and the world's lack of it. In regard to that conflict, absurdism is not just the thesis that nothing matters: things seem to matter to us nonetheless, and the impression cannot be shaken off. Camus's own answer was not despair: he argued that realising the absurd does not justify suicide but calls for revolt, and he ended The Myth of Sisyphus by saying that one must imagine Sisyphus happy.