In the first 40 pages of the Myth of Sisyphus, Camus sets out to define the absurd as well as lay out the main question which he seeks to explore: that of suicide. For Camus, the absurd is not really a thing in itself; rather, it is the product of the relationship between logical human*, who has a desire to find rational meaning in the world, and the world itself, which is fundamentally irrational and will never provide the meaning man so desperately desires. That confrontation between these two and the ensuing disparate gap is itself the absurd. Therefore, it is not the world or human that is absurd, it is their irreconcilable relationship and the struggle between them that is.
Because the absurd is between each human and the world, Camus eschews a more societal approach to the issue in favor of an individual one. Specifically, he is focusing on the “relationship between individual thought and suicide” because he believes that thinking is itself the beginning of the existential process and “society has but little connection with such beginnings” (4-5). As a result, Camus is forced into an investigation of human nature, searching for essentialized features of humans to which he can ascribe the desire for logical meaning (because it is this desire in humans which enables them to enter into conflict with the irrational world and access the notion of the absurd). However, he undermines himself when he cites several other philosophers, from Heidegger to Kierkegaard, who explore similar themes of irrationality and alienation from the world, conceding that “all these experiences agree and confirm one another” (27), because he fails to acknowledge the ways in which society structures our understandings of reason, the world, and the concept of meaning itself.
The very notion of there being meaning in life, as Camus understands it, is structured through the socio-historical culture in which he exists and the philosophical tradition in which he is writing. Tellingly, it is important to note that Camus is only citing European philosophers of the Western tradition, generally operating post-Enlightenment. It is through these thinkers that Camus is examining concepts of thought, meaning, and the world, which guides his view in a way that might be entirely different from someone in a different cultural, religious, and/or social background. For example, the individual is the atomic unit which Camus studies, but other philosophies elsewhere in time and space have argued that community is the base unit from which the individual derives, which could have drastically different implications for the philosophical discussion Camus is having, though that is not to say that there is nothing from Camus that could be applied within that framework.
In arguing that reason is not able to fully explain the order and meaning of the world, Camus explicitly rejects rationalism, which here is referred to as the belief that human reason has the capacity to explain the world/universe. Camus also argues that the absurd relies on two necessary conditions for existence: human desire for logical meaning and the universe’s permanent guarantee to never provide it, and that to explore the absurd you cannot negate either term, as ‘solving’ the problem of the absurd by simply erasing one of the constituent terms is not solving the problem at all but rather eluding it. However, although he claims to reject rationalism, Camus still relies on rationalist logics as the basis of the problem of the absurd: only through a rationalist lens is there the yearning to explain the world and its meaning through logical reasoning accessible to the human mind. A more thorough rejection of rationalism (which remains distinct from the abandonment of all reason) could lead to a negation of one of the terms and elusion of the problem, but does not necessarily constitute fleeing in the same way Kierkegaard’s or Chestov’s answer does because it doesn’t retreat into a recreation of hope through God. Instead, it circumvents hope entirely by recognizing that it is not necessary—it does not matter whether or not it exists because the base desire underpinning that hope (the desire for rational meaning) is not itself present.
*Human is used in the singular (awkwardly) specifically because Camus emphasizes the individuality of the struggle with the irrational world and the rational individual.