Short answers, pulled from the story.
Absurdity derives from the Latin absurdum, meaning "out of tune". Outside music and acoustics, the term came to mean "out of harmony with reason". The Latin root surdus, meaning "deaf", also carries an implication of stupidity.
Plato used absurdity to describe very poor reasoning and the false conclusion that follows from adopting a false premise, a technique known as reductio ad absurdum. In Gorgias, he called the outcome of reasoning from a false assumption an "inevitable absurdity". In Parmenides, he described himself as not using absurd argumentation against himself.
Hobbes' Table of Absurdity is a seven-category classification of linguistic absurdities drawn from Thomas Hobbes' work De Corpore. Each category defines an illegitimate combination of names, such as combining the name of a body with the name of a phantasm. The scholar Aloysius Martinich described all seven categories as reflecting Hobbes' commitment to the new science of Galileo and Harvey.
Camus' absurdism holds that the universe is irrational and meaningless, and that the absurd arises from the collision between the human tendency to seek meaning and the universe's inability to provide it with any certainty. The absurd is not caused by the universe or the human mind separately, but by both existing simultaneously. Camus is most famously associated with absurdism, primarily posthumously.
The absurdity doctrine allows American courts to refuse to apply a law as written when the outcome would be so obviously wrong that, in one formulation, "all mankind would, without hesitation, unite in rejecting the application". Two types exist: the scrivener's error, a simple clerical mistake, and evaluative absurdity, where a provision is grammatically correct but "makes no substantive sense".
The absurdity constant, denoted by the symbol commonly read as bottom or written as an upside-down T, represents falsum, the elementary logical proposition of a constant false value. It is used in formal logic and is implemented directly in several programming languages.