The ridiculous is that which is highly incongruous or inferior, sometimes deliberately so to make people laugh or get their attention, and sometimes unintended so as to be considered laughable and earn or provoke ridicule and derision. It comes from the 1540s Latin “ridiculosus” meaning "laughable”, from “ridiculus” meaning "that which excites laughter”, and from “ridere” meaning "to laugh". "Ridiculous" is an adjective describing "the ridiculous".
In common usage, "ridiculousness" is used as a synonym for absurdity or nonsense. Historic and technical usage of "absurdity" associates it with invalid argumentation and reasoning. Historic and technical usage of "nonsense" associates it with semantics and meaning. While historic and technical usage of "ridiculous" associates it with laughter, superiority, deformity, and incongruity. Reductio ad absurdum is a valid method of argument, while reductio ad ridiculum is invalid. Argument by invective declaration of ridiculous is invalid, while arguments involving declarations of nonsense may summarize a cogent semantic problem with lack or meaning or ambiguity.