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• A logical constant denoting a false proposition in logic, often called "falsum" or "absurdum"
• The bottom element in lattice theory, which also represents absurdum when used for logical semantics
• The bottom type in type theory, which also represents absurdum under the Curry–Howard correspondence

• Mixed radix decoding in the APL programming language

• Perpendicularity of lines in geometry
• Orthogonality in linear algebra
• Independence of random variables in probability theory
• Comparability in order theory
• Coprimality in number theory