Of all the pairs in philosophy that get muddled, this may be the one where the muddle does the most damage. Nihilism and absurdism are treated as synonyms constantly, usually to mean something like "life is pointless." But they are not the same, and the gap between them is precisely the gap between a conclusion that can flatten a person and one designed to lift them back up.
They do share a premise. Both accept that the universe hands us no inherent meaning — no cosmic purpose stamped into existence, no order that makes our lives significant from the outside. Nihilism takes that observation and draws the hard conclusion: therefore nothing truly matters. Values are baseless, purpose is illusion, and in its bleakest forms it offers no reason to prefer any way of living to any other. (Some thinkers soften this into an 'optimistic nihilism' that finds a kind of freedom in the emptiness — but the core move is the same.)
Absurdism's refusalThe same void, a different answer.
Absurdism begins in the identical place and then refuses to end there. Camus fully accepted that the universe is silent, that our craving for meaning meets no reply. But he treated that not as a full stop but as a starting condition — the absurd — to be met with revolt. His famous essay opens by naming suicide the one truly serious philosophical problem, and its whole argument is a sustained no: the honest response to a meaningless universe is not to check out, and not to lie to yourself with false hope, but to live fully, lucidly, and defiantly in spite of it. Where nihilism can trail off into "nothing matters," absurdism answers: nothing matters — so let it matter to you anyway.
That is why Camus reached for Sisyphus, eternally rolling his boulder, and insisted we imagine him happy. A nihilist reading of Sisyphus sees only futility. Camus sees a man who has been handed the most meaningless task imaginable and chooses to own it, to fill his heart with the struggle itself. Whether that is a genuine escape from nihilism or a heroic gesture papering over it is exactly the question the comparison above is built to make you feel.